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THE STORY OF THE STARS. 



DESCRIPTIVE 



ASTRONOMY 



JOEL DORMAN STEELE, Ph.D., 

AUTHOR OF THE FOURTEEN-WEEKS SERIES IN NATURAL SCIENCE. 



"The heavens declare the glory of God; and the firmament showeth 
his handiwork. " — Psalm xix,.- 



K 

J/ 

1 



A. S. BARNES & COMPANY, 

NEW YORK AND CHICAGO. 
1884. 



BY THE SAME AUTHOR. 



HUMAN PHYSIOLOGY. 

HYGIENIC PHYSIOLOGY. 

HYGIENIC PHYSIOLOGY. ABRIDGED 

ZOOLOGY. 

CHEMISTRY. 

PHYSICS. 

GEOLOGY. 

DESCRIPTIVE ASTRONOMY. 

BOTANY. 

Key containing Answers to the Practical Questions 
and Problems. Post-paid, $1.25. 









>A 



Cafivright, 1884. by A. S. Barnes & Co. 



PREFACE TO THE FIRST EDITION. 



T~\UBING the past few years great advances 
-"—^ have been effected in astronomical science. 
Physics has come to the help of Mathematics, and, 
not content with the old question, where the heav- 
enly bodies are, has sought to find out what they are. 
Valuable discoveries have been made concerning 
Meteors, Shooting Stars, the Constitution of the Sun, 
the Motion of the Heavenly Bodies, &c. The investi- 
gations connected with Spectrum Analysis have been 
especially suggestive. On every hand the facts of 
the New Astronomy have been accumulating. Until 
recently, however, they were scattered through many 
expensive books, and were consequently beyond the 
reach of the most of our schools. It has been the 
aim to collect in this little volume the most interest- 
ing features of the larger works. 

Believing that Natural Science is full of fascina- 
tion, the author has sought to weave the story of 
those far-distant worlds into a form that may attract 
the attention and kindle the enthusiasm of the pupil. 

This work is not written for the information of 
scientific men, but for the inspiration of youth. 



VI PREFACE TO THE FIRST EDITION. 

Therefore the pages are not burdened with a multi- 
tude of figures which no memory could retain. 

Mathematical tables and data, Questions for Re- 
view, a very valuable Guide to the Constellations, 
and an Apparatus for Illustrating Precession, are 
given in the Appendix, where they may be useful 
for reference. 

Those persons having a small telescope will find 
valuable assistance in the " List of interesting Ob- 
jects for a common Telescope." The Index contains 
the pronunciation of many difficult names. 

Particular attention is called to the method of 
classifying the measurements of Space, and the 
practical treatment of the subjects of Parallax, Har- 
vest Moon, Eclipses, the Seasons, Phases of the Moon, 
Time, Nebular Hypothesis, Spectrum Analysis, and 
Precession. 

To teachers hitherto compelled to use a cumber- 
some set of charts, it is hoped that the star maps 
here offered will present a welcome substitute. The 
geometrical figures, showing the actual appearance of 
the constellations, will relieve the mind confused with 
the idea of numberless rivers, serpents, and classical 
heroes. Only the brightest stars are given, since in 
practice it is found that pupils remember the general 
outlines alone, while the details are soon forgotten. 

Many of the cuts are copied from the French edi- 
tion of Guillemin's " Heavens." Acknowledgment 



ASTRONOMY. Vll 

for much valuable material is hereby made to this 
excellent work, and also to "Chambers's Astron- 
omy/*' " Newcomb's Astronomy," and Young's "The 
Sun." 

Finally, the author commits this little work to the 
hands of the young, to whose instruction he has con- 
secrated the energies of his life, in the earnest hope 
that, loving Nature in all her varied phases, they 
may come to understand somewhat of the wisdom, 
power, beneficence, and grandeur displayed in the 
Divine harmony of the Universe. 

' ' One God, one law, one element, 

And one far-off Divine event 
To which the whole creation moves. " 



READING REFERENCES. 

Chambers's Astronomy.— Young's The Sun.— Ball's Elements of Astronomy.— 
Newcomb's Popular Astronomy— Lockyer's Spectrum Analysis.— Proctor's Other 
Worlds than Ours, Saturn, The Moon, Poetry of Astronomy, &c. — Delaunay's Cours 
D'Astronomic.— Ilaughton's Manual of Astronomy. — Newcomb and Holden's As- 
tronomy.— Lockyer's Elements of Astronomy.— Norton's Spherical and Physical 
Astronomy. — IlerschcTs Outlines of Astronomy.— Robinson's Astronomy.— Mitch- 
ell's Popular Astronomy. — Arago's Popular Astronomy. — Airy's Lectures on 
Astronomy.— Hind's Solar System, and Introduction to Astronomy. — Lockyer's Ele- 
mentary Lessons in Astronomy. — Proctor's Star Atlas.— Heis's Star Atlas.— Peck's 
Popular Astronomy. — Gillet and Rolfe's Astronomy.— Sharpless and Phillips's As- 
tronomy.— Peabody's Elements of Astronomy.— Schellen's Spectrum Analysis.— 
Winchell's World-Life (excellent reading in connection with the Nebular Hypothe- 
sis).— Flammarion's Wonders of the Heavens.— Guillemin's The Heavens, revised by 
Proctor.— Loomis's Elements of Astronomy.— Proctor's Easy Star Lessons.— Olm- 
steacTs Letters on Astronomy — Routledge's History of Science.— Buckleys History 
of Natural Science.— Williamson's Problems on the Globes.— The Popular Science 
Monthly (1872-1884;. — Rambosson's Histoire Des Astres. 



SUGGESTIONS TO TEACHERS. 



rpHIS work is designed to be recited in the topical method. On hear- 
ing the title of a paragraph, the pupil should be able to draw upon 
the blackboard the diagram, and to state the substance of what is con- 
tained in the book. It will be noticed that the order of topics, in treat 
ing of the planets and also of the constellations, is uniform. If, each 
day, a portion of the class write their topics in full upon the black- 
board, it will be found a valuable exercise in spelling, punctuation, and 
composition. Every point which can be illustrated in the heavens 
should be shown to the class. No description or apparatus can equal 
the reality in the sky. After a constellation has been traced, the pupil 
should be practised in star-map drawing. 

The article on "Celestial Measurements," near the close of the 
work, should be constantly referred to during the term. In the figures, 
and especially in the star-maps, it should be remembered that the right- 
hand side represents the west ; and the left-hand, the east. To obtain 
this idea correctly, the book should, in general, be held up toward the 
southern sky. 

For the purpose of more easily finding the heavenly bodies at any 
time, Whitall's Movable Planisphere is of great service. It may be 
procured of the publishers of this work. A tellurian is invaluable in 
explaining Precession of the Equinoxes, Eclipses, Phases of the Moon, 
etc. Messrs. A. S. Barnes & Co., New York City, furnish a good in- 
strument at a low price. A small telescope, or even an opera-glass, 
will be useful. A good star-map, and as many advanced works upon 
Astronomy as can be secured, should be included in the teacher's 
outfit. 



X SUGGESTIONS TO TEACHERS. 

The pupil should, at the outset, get a distinct idea of the circles 
and planes of the celestial sphere. The subject of angular measure 
ments can easily be made clear in this relation. A circle contains 
360° ; 90° reach from horizon to zenith ; 180 c produce opposition ; 
while smaller distances can be shown in the sky (see pp. 216, 228). 

Never let a pupil recite a lesson, nor answer a question, except it be 
a mere definition, in the language of the book. The text is designed to 
interest and instruct the pupil ; the recitation should afford him an 
opportunity of expressing what he has learned, in his own style and 
words. 

Teachers desiring additional information are advised to read " New- 
comb's Astronomy," Young's " The Sun," Proctor's Works, "Chambers's 
Astronomy," and Ball's " Elements of Astronomy." 



TABLE OF CONTENTS. 



PAGE 

INTRODUCTORY REMARKS 1 



I. INTRODUCTION. 

HISTORY OF ASTRONOMY 5 

SPACE 24 

The Three Systems op Cikcles 26 

The ZoDrAC 31 

II. THE SOLAR SYSTEM 35 

THE SUN 36 

THE PLANETS 55 

Vulcan 71 

Mercury 71 

Venus 77 

The Earth 82 

The Seasons 95 

Precession and Nutation ! . 104 

Refraction, Aberration, and Parallax 112 

The Moon 122 

Eclipses 138 

The Tides 147 

Mars 150 

The Minor Planets 154 

Jupiter . 157 

Saturn 164 

Uranus 170 

Neptune.. . 172 



Xll tABLE OF CONTENTS. 

PAGE 

METEORS AND SHOOTING STARS 175 

COMETS 185 

ZODIACAL LIGHT 196 



III. THE SIDEREAL SYSTEM 201 

THE STARS 203 

THE CONSTELLATIONS 214 

Northern Circumpolar Constellations 214 

Equatorial Constellations 220 

Southern Constellations 238 

DOUBLE STARS, COLORED STARS, VARIABLE STARS, 

CLUSTERS, MAGELLANIC CLOUDS, &c 239 

Nebula 246 

The Milky Way 253 

The Nebular Hypothesis 255 

CELESTIAL CHEMISTRY.— Spectrum Analysis 258 

TIME 263 

CELESTIAL MEASUREMENTS 271 

IV. APPENDIX 289 

Tables 291 

Questions 293 

Guide to the Constellations 313 

Apparatus 317 

List of Interesting Objects Visible with an Ordinary 

Telescope 319 

Index 323 



INTRODUCTORY REMARKS* 

\ STRONOMY (astron, a star ; nomos, a law) treats of the 
AA Heavenly Bodies — the sun, moon, planets, stars, etc., 
and, as our globe is a planet, of the earth also. It 
is, above all others, a science that cultivates the imagination. 
Yet its theories and distances are based upon rigorous mathemat- 
ical demonstrations. Thus the study has at once the beauty of 
poetry and the exactness of Geometry. 

The great dome of the sky, filled with glittering stars, is one 
of the most sublime spectacles in nature. To enjoy this fully, a 
night must be chosen when the air is clear, and the moon is ab- 
sent. We then gaze upon a deep blue, an immense expanse 
studded with stars of varied color and brilliancy. Some shine 
with a vivid light, perpetually changing and twinkling ; others, 
more constant, beam tranquilly and softly upon us ; while many 
just tremble into our sight, like a wave that, struggling to reach 
some far-off land, dies as it touches the shore. 

In the presence of such weird and wondrous beauty, the ten- 
derest sentiments of the heart are aroused. A feeling of awe and 
reverence, of softened melancholy mingled with a thought of 
God, comes over us, and awakens the better nature within us. 
Those far-off lights seem full of meaning to us, could we but 
read their message ; they become real and sentient, and, like the 
soft eyes in pictures, look lovingly and inquiringly upon us. We 
come into communion with another life, and the soul asserts its 
immortality more strongly than ever before, We are humoled 
as we gaze upon the infinity of suns, and strive to comprehend 

* This Introduction is designed merely to furnish suggestive material for conversa- 
tion at the first lesson, preparatory to beginning the study. It is not intended for com- 
mittal. Other topics may be found in the Questions given in the Appendix. 



% INTRODUCTORY REMARKS. 

their enormous distances, and their magnificent retinue of worlds. 
The powers of the mind are aroused, and eager questionings 
crowd upon us. What are those glittering fires 1 What is their 
distance ? Are they worlds like our own % Do living, thinking 
beings dwell upon them ? Are they promiscuously scattered 
through space, or is there a system in the universe % Can we 
ever hope to fathom those mysterious depths, or are they closed 
to us forever 1 

Some of these problems have been solved ; others yet await 
the astronomer whose eye shall be keen enough to read the mys- 
terious scroll of the heavens. Two hundred generations of study 
have revealed to us such startling facts, that we wonder how man 
in his feebleness can grasp so much, see so far, and penetrate so 
deeply into the mysteries of the universe. Astronomy has meas- 
ured the distance of a few stars, and of all the planets ; com- 
puted the mass, size, days, years, seasons, and many physical 
features of the planets ; made a map of the moon ; tracked 
many of the comets in their immense sidereal journeys ; and, at 
last, analyzed the structure of the sun and stars, and announced 
the very elements of which they are composed. 

Observing for several evenings those stars which shine with a 
clear, steady light, we notice that they change their position with 
respect to the others. They are therefore called planets (literally 
icanderers). Others remain immovable, and shine with a shift- 
ing, twinkling light. They are termed the fixed stars, although 
it is now known that they also are in motion — the most rapid of 
any known even to Astronomy — but through such immense 
orbits that they seem to us to be stationary. Then, too, diag- 
onally girdling the heavens, is a whitish, vapory belt — the Milky 
Way. This is composed of multitudes of millions of suns — of 
which our own sun itself is one — so far removed from us that 
their light mingles, and makes only a fleecy whiteness. 
- This magnificent panorama of the heavens is before us, inviting 
our study, and waiting to make known to us the grandest revela- 
tions of science. 



INTRODUCTION 







' 1. 


Among the Chinese. 








2. 


Among the Chaldeans. 








3. 


1 2. 

Among the Grecians. 4 3. 

1 4. 

1,5. 


Thales. 
Anaximander. 
Pythagoras. 

Anaxagoras & Eudoxus. 
Hipparchus. 






4. 


The Egyptians -j 2 ' 


The School at Alexandria 
Ptolemy and his Theory. 






5. 


The Saracens. 






1. History 


6. 


Astrology. 




o 

t— 1 




7. 


The Copernican System. 






8. 


Tycho Brahe. 




o 




9. 


Kepler's Laws. 




o 




10. 


f L 
Galileo J. 2. 


His Telescope. 
His Discoveries. 


(3. 


Their Reception. 


05 




11. 


( a. 

Newton, and the Law J b. 
of Gravitation. i 

( c. 


Laws of Motion. 

Their Application to Moon's 

Phases. 
The Result. 



c 1. Celestial Sphere. 



The Principal Circle, 
f 1. The Hori- J b. The Subord. Circle, 
zon. ) c. Points. 

d. Measurements. 



The Three Sys- 
tems of Cir- 
cles. 



3. The Zodiac 



. The Equi- 
noctial. 



Ecliptic. 



(a. The Principal Circle. 

)b. "" 

U 



The Subord. Circle. 

Points. 

Measurements. 

(a. The Principal Circle 
) b. The Subord. Circle. 
J c. Points. 
( d. Measurements. 




Galileo. Kepler. 



Copernicus. Tycho Brake. Isaac Newton. 



I.-THE HISTORY. 

Astronomy is the most ancient of the sciences. 
The study of the stars is doubtless as old as man 
himself, and hence many of its discoveries date back 
of authentic records, amid the mysteries of tradition. 
In tracing its history, we shall speak only of those 



6 THE HISTORY. 

prominent facts that will enable us to understand its 
progress and glorious achievements. 

The Chinese boast much of their astronomical dis- 
coveries. Indeed, their emperor claims a celestial 
ancestry, and styles himself the Son of the Sun. 
They possess an account of a conjunction of four 
planets and the moon, which occurred in the 25tb 
century before Christ. They have also the first record 
of an eclipse of the sun (b.c. 2128) ; and one of their 
emperors put to death the chief astronomers Ho and 
Hi for failing to announce the solar eclipse of 
2169 B.C. 

The Chaldeans. — The Chaldean shepherds, watch- 
ing their flocks by night under a sky famed for its 
clearness and brilliancy, could not fail to become 
familiar with many of the movements of the heav- 
enly bodies. Their priests were astronomers ; and 
their temples, observatories. When Alexander took 
Babylon (b.c. 331), he found a record of their obser- 
vations reaching back nineteen centuries.* The 
Chaldeans divided the day into hours, invented the 
sun-dial, and discovered the Saros, or Chaldean Pe- 
riod — the length of time in which eclipses of the sun 
and the moon repeat themselves in the same order. 

The Grecians. — Though the Asiatics were patient 
observers, they did not classify their knowledge, and 
lay the basis of a science. This became the work of 
the western mind. 

Thales (b.c. 640-548), one of the seven sages of 

* Many astronomical inscriptions have been found in the ruins of Nineveh. In the 
public library of that city there was a series of about seventy-two volumes, called the 
Observations of Bel. One book treated of the polar star (then Alpha of the Dragon), 
another of Venus, and a third of Mars. The earliest of these records are thought to 
date back as far as 2540 b.c. (See Records of the Past, Vol. I.) 



THE GRECIANS. 7 

Greece, has been styled the Father of Astronomy. 
He taught that the earth is round, and that the moon 
receives her light from the sun. He determined 
when the equinoxes and the solstices occur, and also 
predicted an eclipse of the sun that is famous for 
having terminated a war between the Medes and the 
Lydians. These nations were engaged in a fierce 
battle, but the awe produced by the darkening of the 
sun was so great, that both sides threw down their 
arms and made peace. 

Anaximander (b.c. 610-546) invented the sun-dial, 
and explained the cause of the moon's phases. 

Pythagoras (b.c. 570-500) founded a celebrated 
astronomical school at Crotona, Italy, where were 
educated hundreds of enthusiastic pupils.* He was 
emphatically a dreamer. He conceived a system of 
the universe, in many respects correct ; yet he ad- 
vanced no proof, made few converts to his views, 
and they were soon well-nigh forgotten. 

He held that the sun is the center of the solar sys- 
tem, the planets revolving about it in circular orbits ; 
that the earth rotates daily on its axis, and revolves 
yearly round the sun ; that Venus is both morning 
and evening star ; that the planets are placed at 
intervals corresponding to the scale in music, and 
that they move in harmony, making the "music of 
the spheres," but that this celestial concert is heard 
only by the gods, — the ears of man being too gross 
for such divine melody. He also believed that the 
planets are inhabited, and he even attempted to cal- 
culate the size of the animals in the moon. 

* See Barnes's History of the Ancient Peoples, p. 174. 



8 THE HISTORY. 

Anaxagoras (b.c. 500-428) taught that there is but 
one God, and that the sun is only a fiery globe, and 
should not be worshipped. He attempted to explain 
eclipses and other celestial phenomena by natural 
causes, saying that there is no such thing as chance 
or accident, these being only names for unknown 
laws. For his audacity and impiety, as his country- 
men considered it, he and his family were doomed to 
perpetual banishment. 

Eudoxus, who lived in the fourth century B.C., in- 
vented the theory of the Crystalline Spheres. He 
held that the heavenly bodies are set, like gems, in 
hollow, transparent, crystal globes, which are so 
pure that they do not obstruct our view, while they 
all revolve around the earth ; and that the planets 
are placed in one globe, but have a power of moving 
themselves, under the guidance — as Aristotle taught 
— of a tutelary genius, who resides in each, and rules 
over it as the mind rules over the body. 

Hipparchus, who flourished in the second century 
B.C., has been called the Newton of Antiquity. He 
was the most celebrated of the Greek astronomers. 
He calculated the length of the year to within six min- 
utes, discovered the precession of the equinoxes, and 
made the first catalogue of the stars — 1080 in number. 

The Egyptians. — Egypt, as well as Chaldea, was 
noted for its knowledge of the sciences long before 
they were cultivated in Greece. It was the practice 
of the Greek philosophers, before aspiring to the 
rank of teacher, to travel for years through these 
countries, and gather wisdom at its fountain-head. 
Pythagoras spent thirty years in this kind of study. 






THE EGYPTIANS. 9 

Two hundred years after Pythagoras, the cele- 
brated school of Alexandria was established. * Here 
were concentrated in vast libraries and princely 
halls nearly all the wisdom and learning of the 
world. Here nourished the sciences and arts, under 
the patronage of munificent kings. 

At this school, Ptolemy (a.d. 70), a Grecian, wrote 
his great work, the Almagest, which for fourteen 
centuries was the text-book of astronomers. In this 
work was given what is known as the Ptolemaic 
System. It was founded largely upon the materials 
gathered by previous astronomers, such as Hippar- 
chus, whom we have already mentioned, and Era- 
tosthenes, who computed the size of the earth by the 
means even now considered the best — the measure- 
ment of an arc of the meridian. 

Ptolemaic System. — To the early astronomers, the 
movements of the planets seemed extremely com- 
plex. Venus, for instance, was sometimes seen as 
evening star in the west, and then again as morning 
star in the east. Sometimes she appeared to be 
moving in the same direction as the sun, then, going 
apparently behind the sun, she seemed to pass on 
again in a course directly opposite. At one time, 
she would recede from the sun more and more slowly 
and coyly, until she would appear to be entirely sta- 
tionary ; then she would retrace her steps, and seem 
to meet the sun. 

An attempt was made to account for all these 
facts by an incongruous system of " Cycles and 

* See Barnes's General History, p. 154. 



10 



THE HISTORY. 



epicycles," as it is called.* The advocates of this 
theory assumed that every planet revolves in a 
circle, and that the earth is the fixed center around 
which the sun and the heavenly bodies move. They 
then conceived that a bar, or something equivalent, 
is connected at one end with the earth ; that at some 
part of this bar the sun is attached ; while between 
that and the earth, Venus is fastened — not to the bar 
directly, but to a sort of crank ; and further on, Mer- 
cury is hitched on in the same way. 

In Fig. 3, let A be the earth ; S, the sun ; A B D F, 
the bar (real or imaginary) ; B C, the short bar or 
crank to which Venus is tied ; D E, another bar for 
Mercury ; F G, a fourth bar, with still another short 
crank, at the end of which, H, Mars is attached. 



Fig. 3. 




The Ptolemaic 



Thus they had a complete system. They did not 
exactly understand the nature of these bars — 
whether they were real or only imaginary — but they 
did comprehend their action, as they thought ; and 

* Milton refers to this when he speaks of the heavens as— 
" With centric and eccentric scribbled o'er, 
Cycle and- epicycle, orb in orb." 



THE SARACENS. 11 

so they supposed the bar revolved, carrying the sun 
and planets along in a large circle about the earth ; 
while all the short cranks kept flying around, thus 
sweeping each planet through a smaller circle. 

By this theory, we can see that the planets would 
sometimes go in front of the sun and sometimes 
behind ; and their places were so accurately pre- 
dicted, that the error could not be detected by the 
rude instruments then in use. As soon as a new 
motion of one of the heavenly bodies was discovered, 
a new crank, and of course a new circle, was added 
to account for the fact. Thus the system became 
more and more complicated, until, at last, a com- 
bination of five cranks and circles was necessary to 
make the planet Mars keep pace with the Ptolemaic 
theory. No wonder --that Alfonso, of Castile, a 
celebrated patron of Astronomy, revolted at the 
cumbersome machinery, and cried out, "If I had 
been consulted at the Creation, I could have done 
the thing better than that." 

The Saracens. — After the destruction of the 
library at Alexandria, learning found a home among 
the Mohammedans. Bagdad on the Tigris, and Cor- 
dova on the Guadalquiver became centers of 
science, literature, and art. The treasures of Grecian 
knowledge were eagerly gathered by the Caliphs, 
and we are told that it was not uncommon to see, 
entering the gates of Bagdad, a whole train of 
camels loaded with Greek manuscripts. Gerbert, 
afterward Pope Sylvester II., learned the elements 
of astronomy at the University of Cordova, going, 
after the custom of the time, to Spain for instruc- 



1% THE HISTORY. 

tion, as, formerly, philosophers had gone to Egypt. 
In the Moorish schools, geography was already 
taught by the use of the globe. The first observ- 
atory in Europe was erected at 
Seville (1196). The fragments 
of Saracenic learning that have 
come down to us show that 
the Arabs had constructed astro- 
nomical tables, and endeavored 
to perfect them by means of sys- 
tematic observation of the 
heavens. With the down- 
fall of the Moors, and 
the Eevival of Learning, 
Spain ceased to take the 
lead in scientific study. 





The Giralda, Moorish Observatory at Seville. 



ASTROLOGY. 13 

Astrology.— During all these centuries, astronomy 
owed its development quite as much to a desire of 
foretelling the future, as to a love for science. It 
was the prevalent belief that the stars rule the des- 
tinies of men. The Chaldeans scanned the heavens 
for purposes of divination, so that Chaldean and 
astrologer became synonymous. Tiberius, Emperor 
of Rome, practised astrology. Hippocrates himself, 
the Father of Medicine (b.c. 470), ranked this among 
the most important branches of knowledge for the 
physician. The mysterious study possessed a pecu- 
liar fascination for the Arabians, and they culti- 
vated it assiduously. The Moorish astronomers were 
astrologers as well, and popularized the art in west- 
ern Europe. This superstition reached the height of 
its influence during the Middle Ages. 

The issue of any important undertaking and the 
fortunes of an individual were foretold by the as- 
trologer, who drew up a Horoscope representing the 
position of the sun, moon, and planets at the begin- 
ning of the enterprise, or at the birth of the person. 
It was a complete and complicated system, and con- 
tained regular rules, which guided the interpretation, 
and which were so abstruse as to require years for 
their mastery. Venus foretold love ; Mars, war ; 
the Pleiades (Ple'-ya-dez), storms at sea. 

The ignorant were not the only dupes of this 
visionary system. Lord Bacon believed in it most 
firmly. Kepler, by casting nativities, eked out his 
miserable pittance as royal astronomer. So late even 
as the reign of Charles II. , Lilly, a famous astrolo- 
ger, was called before a committee of the House of 



14 THE HISTORY. 

Commons, to give his opinion on the probable issue 
of some enterprise then under consideration. 

However foolish the system of Astrology may 
have been, it preserved the science of Astronomy 
during the Dark Ages, and prompted to accurate 
observation and diligent study of the heavens. 

The Copernican System. — About the commence- 
ment of the sixteenth century, Copernicus, breaking 
away from the theory of Ptolemy, that was still 
taught in the institutions of learning in Europe, 
revived the theory of Pythagoras. He saw how beau- 
tifully simple is the idea of considering the sun the 
grand center about which revolve the earth and the 
planets. He noticed how constantly, when we are 
riding swiftly, we forget our own motion, and think 
that the trees and fences are gliding by us in the 
contrary direction. He applied this thought to the 
movements of the heavenly bodies, and maintained 
that, instead of all the starry host revolving about 
the earth once in twenty-four hours, the earth simply 
turns on its own axis, and thus produces the ap- 
parent daily revolution of the sun and stars ; while 
the yearly motion of the earth about the sun, trans- 
ferred in the same manner, would account for the 
solar movements. 

Though Copernicus thus simplified the Ptolemaic 
theory, he yet found that the idea of circular orbits 
for the planets would not explain all the phenomena, 
and therefore retained the "cycles and epicycles" 
Alfonso had so heartily condemned. For forty years, 
this illustrious astronomer carried on his observa- 
tions in the upper part of a humble, dilapidated 



KEPLER'S LAWS. 15 

farm-house, through the roof of which he had an 
unobstructed view of the sky. The work containing 
his theory was published just in time to be laid upon 
his death-bed. 

Tycho Brahe, a celebrated Danish astronomer, 
next propounded a modification of the Copernican 
system. He rejected the idea of cycles and epicycles, 
but, influenced by certain passages of Scripture, 
maintained, with Ptolemy, that the earth is the 
center, and that all the heavenly bodies daily re- 
volve about it in circular orbits. Brahe was a noble- 
man of wealth, and, in addition, received large sums 
of money from the government. He erected a mag- 
nificent observatory, and made many beautiful and 
rare instruments. Clad in his robes of state, he 
watched the heavens with the intelligence of a 
philosopher and the splendor of a king. His inde- 
fatigable industry and zeal resulted in the accumu- 
lation of a vast fund of astronomical knowledge, 
which, however, he lacked the ability to apply to any 
further advance in science. 

His pupil, Kepler, saw these facts, and in his fruit- 
ful mind they germinated into three great truths, 
called Kepler's laws. These form one of the most 
precious conquests of the human mind. They are 
the three arches of the bridge over which Astronomy 
crossed the gulf between the Ptolemaic and Coper- 
nican systems. 

Kepler's Laws. — Kepler, taking the investigations 
of his master, Tycho Brahe, determined to find what 
is the exact shape of the orbits of the planets. He 
adopted the Copernican theory — that the sun is the 



16 THE HISTORY. 

center of the system. At that time, all believed the 
orbits to be circular. They reasoned thus : the circle 
is perfect ; it is the most beautiful figure in nature ; 
it has neither beginning nor ending ; therefore, it is 
the only form worthy of God, and He must have 
used it for the orbits of the worlds He has made. 

Imbued with this romantic view, Kepler com- 
menced with a rigorous comparison of the places of 
the planet Mars as observed by Brahe, with the 
places as stated by the best tables that could be com- 
puted on the circular theory. For a time, they 
agreed, but in certain portions of the orbit the obser- 
vations of Brahe would not fit the computed place 
by eight minutes of a degree. Believing that so 
good an astronomer could not be mistaken as to the 
facts, Kepler exclaimed, " Out of these eight minutes 
we will construct a new theory that will explain the 
movements of all planets." 

He resumed his work, and for eight years con- 
tinued to imagine every conceivable hypothesis, and 
then patiently to test it — "hunt it down," as he 
called it. Each in turn proved false, until nineteen 
had been tried. He then determined to abandon the 
circle and to adopt another form. The ellipse sug- 
gested itself to his mind. Let us see how this figure 
is made. 

Attach a thread to two pins, as at FF in the figure ; 
next, move a pencil along with the thread, the latter 
being kept tightly stretched, and the point will mark 
a curve, flattened in proportion to the length of the 
string, — the longer the string, the nearer a circle 
will the figure become. This figure is the ellipse. 



KEPLER S LAWS. 



17 



The two points F F are called the foci (singular, 
focus). We can now understand Kepler's attempt, 
and the triumph which crowned his seventeen years 
of unflagging toil. 




First Law. — With this figure he constructed an 
orbit having the sun at the center, and again fol- 
lowed the planet Mars in its course. But very soon 
there was as great a discrepancy between the ob- 
served and computed places as before. Undismayed 
by this failure, Kepler assumed another hypothesis, 
and determined to place the sun at one of the foci 
of the ellipse. Once more he "hunted down" the 
theory. For a whole year he traced the planet along 
the imaginary orbit, and it did not diverge. The 
truth was discovered at last, and Kepler (1609) an- 
nounced his first great law — 

Planets revolve in ellipses, with the sun at one focus. 



18 THE HISTORY. 

Second Law. — Kepler knew that the planets do 
not move with equal velocity in the different parts 
of their orbits. He next set about establishing some 
law by which this speed could be determined, and 
the place of the planet computed. He drew an 
ellipse, and once more marked the various positions 
of the planet Mars. He soon found that when at 
its perihelion (point nearest the sun) its motion 
is fastest, but when at its aphelion (point furthest 
from the sun) its motion is slowest. Again he 
"hunted down" various hypotheses, until, at last, 

Fig. 6. 




he discovered that though, in going from B to A, the 
planet moves more slowly, and from D to C more 
rapidly, yet the space inclosed between the lines SB 
and SA is equal to that inclosed between SE> and SC. 
Hence the second law — 

A line connecting the center of the earth with the 
center of the sun passes over equal spaces in equal 
times. 

Third Law. — Kepler, not satisfied with the dis- 
covery of these laws, now determined to ascertain if 
there were not some relation existing between the 



GALILEO. 19 

times of the revolutions of the planets about the sun 
and their distances from that body. With the same 
wonderful patience, he took the figures of Tycho 
Brahe, and began to compare them. He tried them 
in every imaginable relation. Next he took their 
squares, then he attempted their cubes. Here was 
the secret ; but he toiled around it, made a blunder, 
and waited for months, until, once more, his patience 
triumphed, and he reached (1618) the third law — 

The squares of the times of revolution of the planets 
about the sun are proportional to the cubes of their 
mean distances from the sun. * 
/ In rapture over the discovery of these three laws, 
so marked by that Divine simplicity which pervades 
all the laws of nature, Kepler exclaimed, "Nothing 
holds me. The die is cast. The book is written, to 
be read now or by posterity, I care not which. It 
may well wait a century for a reader, since God has 
waited six thousand years for an observer, f 

Galileo. — Contemporary with' Kepler was the great 
Florentine philosopher, Galileo. He discovered the 
laws of the pendulum and of falling bodies, as we 
have already learned in Physics. He was, however, 
educated in and believed the Ptolemaic system. A 
disciple of the Copernican theory happening to come 
to Pisa, where Galileo was teaching as professor in 

* For example : The square of Jupiter's period is to the square of Mars's period, as 
the cube of Jupiter's distance is to the cube of Mars's distance ; or, representing the 
earth's time of revolution by P, and her distance from the sun by p, then letting D and 
d represent the same in another planet, we have the proportion P 5 : D 2 : : p 3 : d 3 . 

t Kepler, strangely enough, believed in the " Music of the Spheres." He made 
Saturn and Jupiter take the bass, Mars the tenor, Earth and Venus the counter, and 
Mercury the treble. This shows what a streak of folly or superstition may run through 
the character of the noblest man. However, as Johnson says, a mass of metal may be 
gold, though there be in it a littla vein of tin- 



20 THE HISTORY. 

the University, drew his attention to its simplicity 
and beauty. His clear, discriminating mind per- 
ceived its perfection, and he henceforth advocated it 
with all the ardor of his unconquerable zeal. Soon 
after, he learned that one Jansen, a Dutch watch- 
maker, had invented a contrivance for making dis- 
tant objects appear near. With his profound knowl- 
edge of optics and philosophical instruments, Galileo 
caught the idea, and soon had a telescope completed. 
It was a very simple affair — only a piece of lead pipe 
with a lens set at each end ; but it was destined to 
overthrow the old Ptolemaic theory, and revolu- 
tionize the science of Astronomy. 

Discoveries made with the Telescope. — Galileo 
now examined the moon. He saw her mountains and 
valleys, and watched the dense shadows upon her 
plains. On January 8, 1610, he turned the telescope 
toward Jupiter. Near it he saw three bright stars, 
as he considered them, which were invisible to the 
naked eye. The next night he noticed that they had 
changed their relative positions. Astonished and 
perplexed, he waited three days for a fair night in 
which to resume his observations. The fourth night 
was favorable, and he found the three star" 
again shifted. Night after night he watched them, 
discovered a fourth star, and finally found that they 
were rapidly revolving around Jupiter, each in its 
elliptical orbit, with its own rate of motion, and all 
accompanying the planet in its journey around the 
sun. Here was a miniature Copernican system, hung 
up in the sky for every one to see and examine for 
himself, 



NEWTON. 21 

V Eeception of the Discoveries.— Galileo met with 
the most bitter opposition. Many refused to look 
through the telescope lest they might become victims 
of the philosopher's magic. Some prated of the 
wickedness of digging out valleys in the fair face of 
the moon. Others doggedly clung to the theory they 
had held from their youth. * But the truth of the 
Copernican system was now fully established. Phil- 
osophers gradually adopted this view, and the Ptole- 
maic theory became a relic of the past. 

Newton, a young man of twenty-four years, was 
spending the summer of 1GG6 in the country, on 
account of the plague which prevailed at Cambridge, 
his place of residence. One day, while sitting in a 
garden, an apple chanced to fall to the ground near 
him. Reflecting upon the strange power that causes 
all bodies thus to descend to the earth, and remem- 
bering that this force continues, even when we as- 
cend to the tops of high mountains, the thought oc- 
curred to his mind, "May not this same force extend 
to a great distance out in space ? Does it not reach 
the moon ? " 

Laws of Motion. — To understand the reasoning 
that now occupied the mind of Newton, let us apply 
unlaws of motion as we have learned them in 

. '* As a specimen of the arguments adduced against the new system, the following by 
Sizzi is a fair instance. " There are seven windows in the head, through which the air 
is admitted to the body, to enlighten, to warm, and to nourish it, — two nostrils, two 
eyes, two ears, and one mouth. So in the heavens there are two favorable stars, Jupiter 
and Venus ; two unpropitious, Mars and Saturn ; two luminaries, the Sun and Moon ; 
and Mercury alone, undecided and indifferent. From which, and from many other phe- 
nomena in Nature, such as the seven metals, etc., we gather that the number of planets 
is necessarily seven. Moreover, the satellites are invisible to the naked eye, can exercise 
no influence over the earth, and would be useless, and therefore do not exist. Besides, 
the week is divided into seven days, which are named from the seven planets. Now, if 
we increase the number of planets, this whole system falls to the ground." 



22 THE HISTORY. 

Physics. When a body is set in motion, it will con- 
tinue to move forever in a straight line, unless 
another force is applied. As there is no friction in 
space, the planets do not lose any of their original 
velocity, but move now with the same speed which 
they received at the beginning. But this would 
make them all pass along straight lines, and not cir- 
cular orbits. What causes the curve ? Obviously, 
another force. For example : I throw a stone into 
the air. It does not move in a straight line, but in 
a curve, because the earth constantly bends it down- 
ward. 

Application. — Just so the moon is moving around 
the earth, not in a straight line, but in a curve. Can 
it not be that the earth bends it downward, just as 
it does the stone ? Newton knew that a stone falls 
toAvard the earth sixteen feet the first second. He 
conceived, after a careful study of Kepler's laws, 
that the attraction of the earth diminishes according 
to the square of the distance. He supposed (accord- 
ing to the measurement then received) that a body 
on the surface of the earth is exactly four thousand 
miles from the center. He now applied this imag- 
inary law. Suppose the body is removed four thou- 
sand miles from the surface of the earth, or eight 
thousand miles from the center. Then, as it is twice 
as far from the center, its weight will be diminished 
2 2 , or 4 times. If it were placed 3, 4, 5, 10 times fur- 
ther away, its weight would then decrease 9, 16, 25, 
100 times. If, then, the stone at the surface of the 
earth (four thousand miles from the center) falls 
sixteen feet the first second, at eight thousand miles 



NEWTON. 23 

it would fall only four feet ; at 240,000 miles, or the 
distance of the moon, it would fall only about one- 
twentieth of an inch (exactly .053). 

Next the question arose, "How far does the moon 
fall toward the earth, i. e., bend from a straight line, 
every second ? " For sixteen years, with a patience 
rivaling Kepler's, this philosopher sought to solve the 
problem. He toiled over interminable columns of 
figures, to find how much the moon's path curves 
each second. At last, he reached a result, which was 
nearly, but not quite, exact. Disappointed, he laid 
aside his calculations. Repeatedly he reviewed 
them, but could not find a mistake. At length, 
while in London, he learned of a new and more 
accurate measurement of the distance from the cir- 
cumference to the center of the earth. He hastened 
home, inserted this new value in his calculations, 
and soon found that the result would be correct. 
Overpowered by the thought of the grand truth just 
before him, his hand faltered, and he called upon a 
friend to complete the computation. 

From the moon, Newton passed on to the other 
heavenly bodies, calculating and testing their orbits. 
Finally, he turned his attention to the sun, and, by 
reasoning equally conclusive, proved that the attrac- 
tion of that great central orb compels all the planets 
to revolve about it in elliptical orbits, and holds them 
with an irresistible power in their appointed paths.* 

* " Do not understand me at all as saying there is no mystery about the planets' mo- 
tion. There is just one single mystery, — gravitation ; and it is a very profound one. 
How it is that an atom of matter can attract another atom, no matter how great the 
distance, no matter what intervening substance there may be ; how it will act upon 
it, or at least behave as if it acted upon it, — I do not know, I cannot tell. Whether they 



24 THE HISTORY. 

At last, he announced this grand Law of Gravita- 
tion : Every particle of matter in the universe at- 
tracts every other particle of matter with a force 
directly proportional to its quantity of matter, and 
decreasing as the square of the distance increases. 



II. SPACE. 

We now in imagination pass into space, which 
stretches out in every direction, without bounds or 
measures. We look up to the heavens, and try to 
locate some object among the mazes of the stars. 
Bewildered, we feel the necessity of some system of 
measurement. Let us try to understand the one 
adopted by astronomers. 

The Celestial Sphere. — The blue arch of the sky, 
as it appears to be spread over us, is termed the 
Celestial Sphere. There are two points to be noticed 
here. 

First, that so far distant is this imaginary arch 
from us, that if any two parallel lines from different 
parts of the earth were drawn to this Sphere, they 
would apparently intersect. Of course, this could 
not be the fact ; but the distance is so immense, that 
we are unable to distinguish the little difference of 

are pushed together by means of an intervening ether, or what is the action, I cannot 
understand. It stands with me along with the fact, that, when I will my arm to rise, it 
rises. It is inscrutable. All the explanations that have been given of it seem to me 
merely to darken counsel with words and no understanding. They do not remove the 
difficulty at all. If I were to say what I really believe, it would be, that the motion of 
the spheres of the material universe stand in some such relation to Him in whom all 
things exist, the ever-present and omnipotent God, as the motions of my body do to my 
will : I do not know how, and never expect to know."— Prof. Young. 



SPACE. 25 

four or even eight thousand miles, and the two lines 
would seem to unite : so we must consider this great 
earth as a mere speck or point at the center of the 
Celestial Sphere. 

Second, that we must neglect the entire diameter 
of the earth's orbit, so that if we should draw two 
parallel lines, one from each end of the earth's orbit, 
to the Celestial Sphere, although these lines would 
be nearly 186,000,000 miles apart, yet they would 
appear to pierce the Sphere at the same point ; which 
is to say, that, at that enormous distance, 186,000,000 
miles shrink to a point. Consequently, in all parts 
of the earth, and in every part of the earth's orbit, 
we see the fixed stars in the same place. 

This sphere of stars surrounds the earth on every 
side. In the daytime, we cannot see the stars be- 
cause of the superior light of the sun ; but, with a 
telescope, they can be traced, and an astronomer will 
find certain stars as well at noon as at midnight. 

One half of the sphere is constantly visible to us ; 
and so far distant are the stars, that we see just as 
much of the sphere as we should if the upper part of 
the earth were removed, and we were to stand four 
thousand miles further away, or at the center of the 
earth, where our view would be bounded by a great 
circle of the earth. 

On the concave surface of the Celestial Sphere, 
there are imagined to be drawn three systems of 
circles : the Horizon, the Equinoctial, and the 
Ecliptic System. Each of these has (1) its Prin- 
cipal Circle, (2) its Subordinate Circles, (3) its 
Points, and (4) its Measurements. 



26 



Fig. 7. 
Z 



1. THE HORIZON SYSTEM. 

(a) The Principal Circle is the Rational Horizon. 
This is the great circle whose plane, passing through 
the center of the earth, separates the visible from the 
invisible heavens. The Sensible Horizon is the small 
circle where the earth and the sky seem to meet : it 
is parallel to the rational horizon, but distant from 
it the semi-diameter of the earth. No two places 
have the same sensible horizon : any two, on opposite 
sides of the earth, have the same rational horizon. 

(b) The Subordinate Cir- 
cles are the Prime Verti- 
cal circle, and the Merid- 
ian. A vertical circle is 
one passing through the 
poles of the horizon (ze- 
nith, and nadir). The 
Prime Vertical is a verti- 
cal circle passing through 
the East and West points. 
The Meridian is a vertical 
circle passing through the 
North and South points. 

(c) The Points are the 
Zenith, the Nadir, and the 

K, S., E., and W. points. The Zenith is the point 
directly overhead, and the Nadir, the one directly 
underfoot. They are also the poles of the horizon— 
i. e., the points where the axis of the horizon pierces 
the Celestial Sphere. The N., S., E., and W. points 
are familiar. 



K \ s N 

^ i^ 

\ \ / y? 



~E, center of earth ; Z, zenith; Z', nadir; 
PP', axis of earth; HAH', horizon; S, a 
star ; ZSZ', vertical circle passing through 
S; AS, altitude of star ; ZS, zenith 
of star; H' 'A, azimuth of star. 



THE HORIZON SYSTEM. 2? 

(d) The Measurements are Azimuth, Amplitude, 
Altitude, and Zenith distance. 

Azimuth is the distance from the meridian, meas- 
ured east or west, on the horizon, to a vertical circle 
passing through the object. 

Amplitude (the complement of Azimuth) is the 
distance from the Prime Vertical, measured on the 
horizon, north or south. 

Altitude is the distance from the horizon, meas- 
ured on a vertical circle, toward the zenith. 

Zenith Distance (the complement of Altitude) is 
the distance from the zenith, measured on a vertical 
circle, toward the horizon. 

The Horizon system is one commonly used in 
observations with Mural Circles, and Transit Instru- 
ments. 

2. THE EQUINOCTIAL SYSTEM. 

(a) The Principal Circle is the Equinoctial. This 
is the Celestial Equator, or the earth's equator ex- 
tended to the Celestial Sphere. At all places between 
the equator and the pole, the celestial equator is in- 
clined to the horizon at an angle equal to the dis- 
tance of the zenith of the place from the pole. * 

(b) The Subordinate Circles are the Hour Circles 
(Right Ascension Meridians), the Colures, and the 
Declination Parallels. 

t The latitude of a place is its distance from the equator, and this equals the distance 
of the zenith of the place from the equinoctial. Hence, having given the latitude of a 
place, to find the height of the celestial equator above its horizon, subtract the latitude 
from 90=, and the remainder is the required angular distance. In like manner, the lati- 
tude subtracted from 90= gives the co-latitude of the place— the complement of the 
latitude. . . 



28 SPACE. 

The Hour Circles are thus located. The Equi- 
noctial is divided into 360°, equal to twenty-four 
hours of motion — thus making 15° equal to one hour 
of motion. Through these divisions run twenty-four 
meridians, each constituting an hour of motion 
(time) or 15° of space. The Hour Circles may be 
conceived as meridians of terrestrial longitude (15° 
apart) extended to* the Celestial Sphere. 

The Colures are two principal meridians ; the 
Equinoctial Colure is the meridian passing through 
the equinoxes ; the Solstitial Colure is the meridian 
passing through the solstitial points. 

The Declination Parallels are small circles 
parallel to the Equinoctial ; or they may be conceived 
as the parallels of terrestrial latitude extended to the 
Celestial Sphere. 

(c) The Points are the Celestial Poles, and the 
Equinoxes. 

The Celestial Poles are the points where the 
axis of the earth extended pierces the Celestial 
Sphere, and are the extremities of the celestial axis, 
as the poles of the earth are the extremities of the 
earth's axis. The North Pole is marked very nearly 
by the North Star, and every direction from that is 
reckoned south, and every direction toward that is 
reckoned north, however it may conflict with our 
ideas of the points of the compass. 

The Equinoxes are the points where the Equinoc- 
tial and the Ecliptic (the sun's apparent path through 
the heavens) intersect. 

(d) The Measurements are Eight Ascension (R. A.), 
Declination, and Polar Distance. 



THE EQUINOCTIAL SYSTEM. 29 

Right Ascension is distance from the Vernal 
Equinox, measured on the equinoctial eastward 
to the meridian which passes through the body, 
K. A. corresponds to terrestrial longitude, and may 
extend to 360° East, instead of 180° as on the earth. 
R. A. is never measured westward. The starting 
point is the meridian passing through the vernal 
equinox, as the meridian passing through Green- 
wich is the point from which terrestrial longitude is 
measured. 

Declination is distance from the equinoctial, 
measured on any Hour Circle or meridian north or 
south. It corresponds to terrestrial latitude. 

Polar Distance (the complement of Declination) 
is the distance from either Pole, measured on an 
Hour Circle. 

The Equinoctial System is largely used by modern 
astronomers, and accompanies the Equatorial Tele- 
scope, Sidereal Clock, and Chronographs of the best 
Observatories. 

3. THE ECLIPTIC SYSTEM. 

(a) The Principal Circle is the Ecliptic. This is 
the apparent path of the sun in the heavens. It is 
inclined to the equinoctial 23£° (23° 27' 15", Jan. 1, 
1884), which measures the inclination of the Earth's 
Equator to its orbit, and is called the obliquity of the 
ecliptic. (See p. 58.) 

The inclination of the ecliptic to the horizon, unlike 
that of the equinoctial, varies at different times of 
the year. The angle that the ecliptic makes with 
the horizon is greatest when the vernal equinox is 



30 SPACE. 

on the western horizon and the autumnal on the 
eastern ; it is least when the vernal equinox is on 
the eastern horizon and the autumnal on the western. * 

(b) The Subordinate Circles are Circles of Celestial 
Longitude, and Parallels of Celestial Latitude. 

The Circles of Celestial Longitude are now 
seldom employed. They are measured on the Eclip- 
tic, as circles of Right Ascension (R. A.) are meas- 
ured on the Equinoctial. 

The Parallels of Celestial Latitude are little 
used. They are small circles drawn parallel to the 
ecliptic, as parallels of declination are drawn parallel 
to the equinoctial. 

(c) The Points are the Poles of the Ecliptic, the 
Equinoxes, and the Solstices. 

The Poles of the Ecliptic are the points where the 
axis of the earth's orbit meets the Celestial Sphere. 

The Equinoxes are the points where the ecliptic 
intersects the equinoctial. The place where the sun 
crosses the equinoctial f in going north, which occurs 
about the 21st of March, is called the Vernal Equinox. 
The place where the sun crosses the equinoctial in 
going south, which occurs about the 21st of Septem- 
ber, is called the Autumnal Equinox. The Solstices 
are the two points of the ecliptic most distant from 
the Equator ; or they may be considered to mark the 
sun's furthest declination north and south of the 
equinoctial. The Summer Solstice occurs about the 

* In the former instance, the angle is equal to the co-latitude, plus 23i° (the inclina- 
tion of the ecliptic to the equinoctial) ; and, in the latter, the co-latitude minus 23^ \ 
Thus, at the latitude of New York, it varies from 90° — 41° + 23J° = 72i ° ; to 90° — 
41° — 23£° == 25|\ In the one case, the summer solstice is on the meridian of the 
place, and, in the other, the winter. 

t " This is commonly called ' crossing the line.' " 



THE ECLIPTIC SYSTEM. 31 

21st of June ; the Winter Solstice occurs about the 
21st of December. 

(d) The Measurements are Celestial Longitude and 
Latitude. 

Celestial Longitude is distance from the Vernal 
Equinox measured on the ecliptic, eastward. 

Celestial Latitude is distance from the ecliptic 
measured on a Subordinate Circle, north or south. 

THE ZODIAC. 
A belt of the Celestial Sphere, 8° on each side of 
the ecliptic, is styled the Zodiac. This is of very 
high antiquity, having been in use among the 
ancient Hindoos and Egyptians. The Zodiac is 
divided into twelve equal parts — of 30° each — called 
Signs, to each of which a fanciful name is given. 
The following are the names of the 

SIGNS OF THE ZODIAC. 



Aries V 

Taurus & 

Gemini n 

Cancer 25 

Leo £1 

Virgo 1TK 



Libra =2: 

Scorpio 1T[ 

Sagittarius $ 

Capricornus Y? 

Aquarius OOC 

Pisces )£ 



" The first, T, indicates the horns of the Earn ; the 
second, « , the head and horns of the Bull ; the barb 
attached to a sort of letter, ni , designates the Scor- 
pion ; the arrow, I , sufficiently points to Sagitta- 
rius ; Y? is formed from the Greek letters, rp, the two 
first letters of rpdyog, a goat. Finally, a balance, 
the flowing of water, and two fishes, tied by a string, 
may be imagined in =*==, ox, and X, the signs of Libra, 
Aquarius, and Pisces." (See pp. 210, 295.) 



32 PRACTICAL QUESTIONS. 



PRACTICAL QUESTIONS. 

1. How high is the North Star above your horizon ? 

2. What is the sun's right ascension at the autumnal equinox ? At the 
vernal equinox ? 

3. What was the first discovery made by the telescope ? 

4. How high above the horizon of any place are the equinoctial points 
when they pass the meridian ? 

5. Jupiter revolves around the sun in 12 of our years. Assuming the 
earth's distance from the sun to be 93,000,000 miles, compute Jupiter's dis- 
tance by applying Kepler's third law. 

6. The latitude of Albany is 42° 39' N ; what is the sun's meridian 
altitude at that place when it is in the celestial equator? 

7. What is the co- latitude of a place ? 

8. What is the declination of the zenith of the place in which you 
reside ? 

9. Why are the stars generally invisible by day ? 

10. Why is the ecliptic so called ? 

11. Who first taught that the earth is round ? 

12. What is Astrology ? 

13. How can we distinguish the fixed stars from the planets ? 

14. How long was the Ptolemaic System accepted ? 

15. In what respect did the Copernican System differ from the one now 
received ? 

16. For what is Astronomy indebted to Galileo ? To Newton ? 

17. What is the amount of the obliquity of the ecliptic ? 

18. Define Zenith. Nadir. Azimuth. Altitude. Equinoctial. Right 
Ascension. Declination. Equinox. Ecliptic. Colure. Solstice. Polar 
distance. Zenith distance. The Zodiac. 

19. If the R. A. of the sun be 80°, state in what sign he is then located ? 
160° ? 280° ? 

20. Why does the angle which the ecliptic makes with the horizon vary ? 

21. Why is the angle which the celestial equator makes with the horizon 
constant ? 



II. 
THE SOLAR SYSTEM. 



" In them hath He set a tabernacle for the sun" 

" This world was once a fluid haze of light, 
Till toward the center set the starry tides 
And eddied into suns, that wheeling cast 
The planets''' — Tennyson. 



w 

Ul 

o 

W 

W 



Distance. 
2. Light & Heat. 
Apparent Size. 

4. Real Dimen- 

sions. 

5. Solar Spots . . . 



, Physical Con- 
stitution 



-Introduction. . 



Mercury -f ; 



II. The Planets. 



f a. Discovery. 

b. Number and Location. 

c. Size. 

d. Constituents, 
c. Motion across Disk. 
/. Change in Rate. 
g. Prove the Rotation of Sun. 
h. Synodic and Sidereal Rotation 
i. Path of Spots. 
,;'. Individual Motion, 
fc. Change in Form. 
I. Periodicity of Spots. 
m. Planetary Influence. 
n. Influence on Terrestrial Heat, etc, 
o. Heat of Spots. 
p. Depression of Spots. 
q. Brightness of Spots. 
r. Faeulae, rice-grains, etc. 

\ a. Wilson's Theory. 
1 b. Present Theory (Kirchholf's). 
How Solar Heat is Produced. 

' a. Common Characteristics. 

b. Comparison of Planets. 

c. Properties of the Ellipse. 

d. Planetary Orbits. 

e. Comparative Size of Planets. 
•/. Conjunction of. 
g. Are Planets Inhabited ? 

. h to p. Division of Planets, etc. 
1. Vulcan. 

f a. Description. 

| b. Motion in Space. 
c. Distance from Earth. 

i d. Dimensions. 

I c. Seasons. 

I /. Telescopic Features. 

3. Venus Repeat same Analysis as of Mercury. 

fa. Dimensions. 

b. Rotundity. 

c. Apparent & Real Motion. 
f \. Diurnal Mo= 

tion of Sun. 
| 2. Unequal rate 
of Motion. 

3. Orbits of 
Stars. 

1 4. Unequal Ve- 
1 locitiesofStars. 
| 5. Appearance 
^ of Stars, etc. 

1. Change in 
appearance of 
heavens. 

2. Yearly path 
of Sun. 
3.MovesN.&S. 

4. Change of 
Seasons, etc. 20 
points under 
this topic. 

/. Precession of Equinoxes. 
g. Nutation. 

h. Refraction & Aberration. 
i. Parallax. 
Mercury. 



\ 4. The Earth . 



0. Motion. 

h. Dimens'ns. 

c. Librations. 

d. L'g't&H't. 
c. Cen.ofGrav. 
/. Atmosph're. 
g. Lunarians. 
h. Earth-shine. 
i. Phases. 

/'. Harv'stM'n. 
it. Wet Moon. 

1. Nodes. 

m. Occulat'n. 
n. Seasons, 
o. Telescopic 
Features. 



d. Diurn'l Mo- 
tion of ■{ 
Earth. 



Yearly Mo- 
tion of 
Sun : its 
Conse- 
quences. 



5. Mars Same Analysis 

G. The Minor Planets. 

7. Jupiter . v .ame Analysis as Mercury. 

8. Saturn " ''• 

9. Uranus " 

V.10. Neptune " " 

III. Meteors, and Shooting Stars ) The subjects of the paragraphs may be inserted 

IV. Comets - by the pupil, to complete these analyses, at 

V. The Zodiacal Light ) the pleasure of the teacher. 



THE SOLAR SYSTEM. 



INTRODUCTION. 

THE Solar System is mainly comprised within 
the limits of the Zodiac. It consists of — 

1. The Sun — the center. 

2. The major planets — Vulcan (undetermined), Mercury, Venus, 

Earth, Mars, Jupiter, Saturn, Uranus, Xeptune. 

3. The minor planets, at present (1884) two hundred and thirty- 

seven in number. 

4. The satellites, or moons, twenty in number, which revolve around 

the different planets. 

5. Meteors and shooting-stars. 

6. Thirteen comets, which have now been found, by a second re- 

turn, to move, like the planets, in elliptic paths, and to 
revisit the sun periodically. 

7. The Zodiacal Light. 

How we are to imagine the solar system to cur- 
selves. — We are to think of it as suspended in space ; 
being held up, not by any visible object, but in 
accordance with the law of Universal Gravitation 
discovered by Newton, whereby each planet attracts 
every other planet and is in turn attracted by all. 

First, the Sun, a great central globe, so vast as to 
overcome the attraction of all the planets, and com- 
pel them to circle around him : next, the planets, 



36 THE SOLAR SYSTEM. 

each turning on its axis while it flies around the sun 
in an elliptical orbit ; then, accompanying these, the 
satellites, each revolving about its own planet, while 
all whirl in a dizzy waltz about the central orb ; 
next, the comets, rushing across the planetary 
orbits at irregular intervals of time and space ; and 
finally, shooting-stars and meteors darting hither 
and thither, interweaving all in apparently inex- 
tricable confusion. 

To make the picture more wonderful still, every 
member is flying with an inconceivable velocity, and 
yet with such accuracy that the solar system is the 
most perfect timepiece known. 



I. THE SUN. 

Sign, ©, a buckler with its boss. 

Distance. — The sun's average distance from the 
earth is nearly 93,000,000 miles.* Since the earth's 
orbit is elliptical, and the sun is situated at one of its 
foci, the earth is 3,000,000 miles further from the sun 
in aphelion than in perihelion. 

* The sun's distance from the earth is determined, as we shall learn hereafter (see 
Celestial Measurements), by means of the solar parallax. In the former editions of this 
work, the parallax of 8". 94— deduced principally from observations upon the planet Mars 
in lSi32— was accepted. This gave a solar distance of about 91 £ million miles, and hi 
been in general use among astronomers until recently. The observations of the last few 
years have, however, shown that the true parallax is smaller, and that the sun is a 
little further off than was supposed. Astronomers are not fully agreed upon the exact 
parallax that should be adopted, but there seems to be a general converging of opinion 
toward S".S0 as being, if not the exact parallax, at least as near it as we are able at 
present to come. This new determination of the solar parallax renders necessary a cor- 
responding change in the planetary distances, etc., as the sun's distance is the unit used 
by astronomers in making all celestial measurements. In this chapter, the author has 
followed the data given by Prof. Young in his work upon the Sun, as being the most 
recent and authoritative view. (See p. 280.) 



THE SUN. 37 

As we attempt to locate the heavenly bodies in 
space, we are startled by the enormous figures em- 
ployed. The first number, 93,000,000 miles, is far 
beyond our grasp. Let us, however, try to compre- 
hend it. * If there were air to convey a sound from 
the sun to the earth, and a noise could be made loud 
enough to pass that distance, it would require over 
fourteen years for it to come to us. Suppose a rail- 
road could be built to the sun. An express-train, 
traveling day and night, at the rate of thirty miles 
an hour, would require 352 years to reach its destina- 
tion. Ten generations would be born and would die ; 
the young men would become gray-haired, and their 
great-grandchildren would forget the story of the 
beginning of that wonderful journey, and would 
read it in history, as we now read of Queen Elizabeth 
or of Shakspere ; the eleventh generation* would 
see the solar station at the end of the route. Yet 
this enormous distance of 93,000,000 miles is used as 
the unit for expressing celestial distances, — as the 
foot-rule for measuring space ; and astronomers 
speak of so many times the sun's distance as we 
speak of so many feet or inches. 

The Light of the Sun is equal to 5,563 wax-candles 
held at a distance of one foot from the eye. It 
would require 600,000 full-moons to produce a day as 
brilliant as one of cloudless sunshine, f 

* If a babe were born with an arm long enough to reach the sun, and should touch 
that fiery globe, the infant would grow to manhood and to old age and finally die, before 
the sensation could traverse the nerve to his brain, and he feel the burn. 

t According to Langley, the sun is blue, and to the inhabitants of other worlds may 
shine as a bluer star than Vega. The light from different parts of the solar disk, how- 
ever, varies in color : while that from the center has a decidedly-blue tint, that from the 
edge is of a chocolate hue. This difference is probably owing to the fact that t^ie latter 



38 THE SOLAR SYSTEM. 

The Heat of the Sun. — The amount of heat we 
receive annually is sufficient to melt a layer of ice 
110 feet thick, extending over the whole earth.* Yet 
the sunbeam is only -3 o o 1 ? oo P ar ^ as intense as it is at 
the surface of the sun. Moreover, the heat and light 
stream off into space equally in every direction. Of 
this vast flood, only one twenty-three-hundred- 
millionth part reaches the earth. 

If the heat of the sun were produced by the burn- 
ing of coal, it would require a layer sixteen feet in 
thickness, extending over its whole surface, to feed 
the flame a single hour. Were the sun a solid body 
of coal, it would burn up at this rate in forty-six 
centuries. Sir John Herschel says that if a solid 
cylinder of ice 45 miles in diameter and 200,000 miles 
long were plunged, end first, into the sun, it would 
melt in a second of time. 

Apparent Size. — The sun appears to be a little over 
half a degree in diameter, so that 337 solar disks, 
laid side by side, would make a half-circle of the 
celestial sphere. It seems a trifle larger to us in 
winter than in summer, as we are 3,000,000 miles 
nearer it. If we represent the luminous surface of 
the sun when at its average (mean) distance by 1,000, 
the same surface will be represented when in aphe- 
lion (July) by 967, and when in perihelion (January) 
by 1,034. 



passes through a greater thickness of the solar atmosphere, while our own atmosphere 
does its part in strangling the blue rays of the sunlight, the red rays filtering through 
with little loss. 

* Recent experiments by Langley seem to increase this estimate to that of a sheet 
of ice 300 feet thick. 



THE SUN. 



39 



Dimensions.— Its diameter is about 866,000 miles.* 
Let us try to understand this amount by comparison. 

A mountain upon the surface of the sun, to bear 
the same proportion to the globe itself as the loftiest 
peak of the Himalayas does to the earth, would need 
to be about 600 miles high. 

Again : Suppose the sun were hollow, and the 
earth, as in the cut (Fig. 8), placed at the center, not 



Fig. 




only would there be room for the moon to revolve in 
its regular orbit within the shell, but that would 



* Pythagoras, whose theory of the universe was in so many respects very like the 
one we receive, believed the sun to be 44,000 miles from the earth, and seventy-five 
miles in diameter. 



40 THE SOLAR SYSTEM. 

stretch off in every direction nearly 200,000 miles 
beyond. 

Its volume is 1,300,000 times that of the earth — 
i. e., it would take 1,300,000 earths to make a globe 
the size of the sun. Its mass is 750 times that of 
all the planets and moons in the solar system, and 
330,000 times that of the ear Hi. Its weight may be 
expressed in tons, thus : 

1,910,278,070,000,000,000,000,000,000.* 

The Density of the sun is only about one-fourth 
that of the earth, or 1.41 that of water, so that the 
weight of a body transferred from the earth to the 
sun would not be increased in proportion to the com- 
parative size of the two. On account also of the vast 
size of the sun, its surface is so far from its center 
that the attraction is largely diminished, since that 
decreases, we remember, as the square of the dis- 
tance. However, a man weighing at the earth's 
equator 150 lbs., at the sun's equator would weigh 
about two tons, — a force of attraction that would in- 
stantly crush him. At the earth's equator, a stone 
falls 16 feet the first second ; at the sun's equator, it 
would fall 444 feet, f 

Telescopic Appearance of the Sun : Sun Spots. — 
We may sometimes examine the sun at early morn- 
ing or late in the afternoon with the naked eye, and 

* This number is meaningless to our imagination, but yet it represents a force of 
attraction that holds our own earth and all the planets steadily in their places; while it 
fills the mind with an indescribable awe as we think of that Being who " made the sun, 
and holds it in the very palm of His hand." 

t A singular consequence of this has been suggested. "A cannon-ball could be 
thrown only a short distance, since it would pass through a path of great curvature, and 
would fall to the sun within a few yards of the gun." 



THE SUN. 



41 



at midday by using a smoked glass. The disk will 
appear distinct and circular, and with no spot to dim 
its brightness. If we use a telescope of moderate 



Fig. 9. 




The Sun seen through a Telescope. 



power, taking the precaution to shield the eye with 
a colored eye-piece, we shall find the surface of the 
sun sprinkled with irregular spots (Fig. 9).* 



* The natural purity of the sun seems to have been formerly an article of faith among 
astronomers, and therefore on no account to be called in question. Schemer, it is said, 
having reported to his superior that he had seen spots on the sun's face, was abruptly 
dismissed with these remarks : " I have read Aristotle's writings from end to end many 
times, and I assure you I do not find anything in them similar to that which you men- 
tion. Go, my son, tranquillize yourself ; be assured that what you take for spots are the 
faults of your glasses or your own eyes." 



42 THE SOLAR SYSTEM. 

Discovery of the Solar Spots.— The solar spots 
seem to have been noticed as early as 807 a.d., al- 
though the telescope was not invented until 1610, 
and Galileo is considered to have discovered them in 
the following year. * 

Number and Location. — Sometimes, but rarely, 
the sun's disk is clear. During a period of ten years, 
observations were made on 1982 days, on 372 of 
which there were no spots seen. As many as two 
hundred spots have been noticed at one time. They 
are mostly found in two belts, one on each side of 
the equator, within not less than 10° nor more than 
30° of latitude. They seem to herd together, — the 
length of the straggling group being generally par- 
allel to the equator. 

Size of the Spots.— It is not uncommon to find a 
spot with a surface larger than that of the earth. 
Schroter measured one more than 29,000 miles in 
diameter. Sir J. W. Herschel calculated that one 
which he saw was 50,000 miles in diameter. In 
1843, one was seen which was 75,000 miles across, 
and was visible to the naked eye for an entire week, f 
On the day of the eclipse in 1858, a spot over 108,000 
miles broad was distinctly seen, and attracted gen- 
eral attention in this country. In 1839, Captain Davis 
saw one which he computed was 180,000 miles long, 
and had an area of twenty -four billion square miles. 

If these are deep openings in the luminous atmos- 

* We read in the log-book of the good ship Richard of Arundell, on a voyage, in 
1590, to the coast of Guinea, that " on the 7, at the going downe of the sunne, we saw a 
great black spot in the sunne ; and the 8 day, both at rising and setting, we saw the like, 
—which spot to me seeming was about the bignesse of a shilling, being in 5 degrees of 
latitude, and still there came a great billow out of the souther board." 

t 1" on the sun's surface = 450.3 miles. This spot was 2 / 47" across (Schwdbe). 



THE SUN. 



43 



Fig. 10. 



phere of the sun, what an abyss must that be at " the 
bottom of which our earth could lie like a boulder in 
the crater of a volcano ! " 

Spots Consist of Distinct Parts. — From the ac- 
companying repre- 
sentation, it will be 
seen that the spots 
generally consist 
of one or more dark 
portions called the 
umbra, and around 
that a grayish por- 
tion styled the pe- 
numbra (pene, al- 
most, and umbra, 
black). Sometimes, 
however, umbrae 
appear without a 
penumbra, and vice 
versa. The umbra itself has generally a dense black 
center, called the nucleus. Besides this, the umbra 
is sometimes divided by luminous bridges. 

Spots are in Motion. — The spots change from day 
to day ; but all have a common movement. About 
fourteen days are required for a spot to pass across 
the disk of the sun from the eastern side, or limb, to 
the western ; in fourteen days, it reappears, changed 
in form perhaps, but generally recognizable. 

Spots apparently Change their Speed and Form 

AS THEY PASS ACROSS THE DlSK. — A Spot is Seen Oil 

the eastern limb ; day by day it progresses, with a 
gradually-increasing rapidity, until it reaches the 




Sun-Spots. 



a 



THE SOLAR SYSTEM. 



center ; it then slowly loses its rapidity, and finally 
disappears on the western limb. The diagram illus- 
trates the apparent change which takes place in the 
form. Suppose at first the spot is of an oval shape ; 
as it approaches the center it apparently widens and 
becomes circular. Having passed that point, it be- 
comes more and more oval until it disappears. 



Fig. 11. 




This change in the Spots proves the Sun's Kota- 
tion on its Axis. — These changes can be accounted 
for only on the supposition that the sun rotates on 
its axis : indeed, they are the precise effects which 
the laws of perspective demand in that case. About 
twenty-seven days elapse from the appearance of a 
spot on the eastern limb before it is seen a second 
time. During this period the earth has gone forward 
in its orbit, so that the location of the observer is 
changed ; allowing for this, the sun's time of rotation 
at the equator is about twenty-five days (25 d., 8 h., 
10 m. : Langier). 



THE SUN. 



45 



Curiously enough, the equatorial regions move 
more rapidly, and complete a rotation in less time, 
than the rest of the sun. While a spot near the 
equator performs a rotation in twenty-seven days, 
one situated half- _. f _ 

l ig. 12. 

way to either pole, 
requires nearly 
twenty -eight days. 
Synodic and side- 
real REVOLUTIONS 
OF THE SPOTS. — We 

can easily under- 
stand why we make 
an allowance for 
the motion of the 
earth in its orbit. 
Suppose a solar spot 
at a, on a line pass- 
ing from the center 
of the earth to the 
center of the sun. 
For the spot to pass 
around the sun and 
come into that same 
position again, requires about twenty-seven days. 
But, during this time, the earth has passed on from 
T to T. The spot has not only traveled around to 
a again, but also beyond that to a, or the distance 
from a to a' more than an entire revolution. To do 
this, requires about two days. A revolution from a 
around to a' is called a synodic, and one from a 
around to a again is called a sidereal, revolution. 




Synodic and Sidereal Revolutions. 



46 the solar system. 

Spots do not always move in straight lines.— 
Sometimes their path curves toward the north, and 






March. June. September. 

sometimes toward the south, as in the figure. This 
can be explained only on the supposition that the 
sun's axis is inclined to the ecliptic (7° 15'). 

Spots have a motion of their own. — Besides the 
motion already named as assigned to the sun's rota- 
tion, nearly every spot seems to have an individual 
motion. Some spots circle about in small elliptical 
paths, often quite regularly for weeks and even 
months. Immense cyclones occasionally pass over 
the surface with fearful rapidity, producing rotation 
and sudden changes in the spots. At other times, 
however, the spots seem ''to set sail and move 
across the disk of the sun like gondolas over a silver 
sea." 

Spots change their real form. — Spots break out 
and then disappear under the eye of the astronomer. 
Wollaston saw one that seemed to be shattered like 
a fragment of ice when it is thrown on a frozen 
surface, breaking into pieces, and sliding off in 
every direction. Sometimes one divides itself into 
several nuclei, while again several nuclei combine 



the su: 



47 



into a single nucleus. Occasionally a spot will re- 
main for six or eight rotations, while often it will 
last scarcely half an hour. Sir W. Herschel relates 




Solar Cyclone, May 5th, 1857. (Secchi.) 

that, when examining a spot through his telescope, 
he turned away for a moment, and on looking back 
it was gone. 

Appearance of the spots is periodical.* — It is 
a remarkable fact that the number of spots increases 
and diminishes through a regular interval of about 
11.11 years. These periodic variations are closely 
connected with similar variations in the aurora 
and magnetic earth-currents which interfere with 
the telegraph. 

Are the spots influenced by the planets ? — 



* The regular increase and diminution in the number of the spots was discovered 
by Schwabe of Prussia, who watched the sun so carefully that it is said " for thirty years 
the sun never appeared above the horizon without being confronted by his imperturbable 
telescope." 



48 THE SOLAR SYSTEM. 

Many astronomers of high standing believe that the 
solar spots are especially sensitive to the approach 
of Mercury and Venus, on account of their nearness, 
and of Jupiter, because of its size ; that the area of 
the spots exposed to view from the earth is uniformly 
greatest when any two of the larger planets come 
into line with the sun ; and that when both Venus 
and Jupiter are on the side of the sun opposite to us, 
the spots are much larger than when Venus alone is 
in that position. Most authorities, however, doubt 
the accuracy of these observations, and deny this 
planetary influence altogether. 

Spots do not influence fruitfulness of the 
season. — Herschel first advanced the idea that years 
of abundant spots would be years also of plentiful 
harvest. This is not now generally received. What 
two years could be more dissimilar than 1859 and 
1860 ? Both abounded in solar spots, yet, in Europe, 
one was a fruitful year and the other one of almost 
famine. Whether the spots influence the weather is 
still a mooted question. 

Spots are cooler than the surrounding sur- 
face. — It seems that the breaking out of a spot sen- 
sibly diminishes the temperature of that portion of 
the sun's disk. The faculae, on the other hand, do 
not increase the temperature (Secchi). 

Spots are depressions. — Careful observations 
show that, in general, the "floor," so to speak, of the 
umbra is sunk from two to six thousand miles below 
the level of the luminous surface (Young). 

Comparative brightness of spots and sun. — 
If we represent the ordinary brightness of the 



THE SUN. 



Fig. 15. 



49 



^ 



Fig. 16. 



Photographic View of S2oots and Faculce. 

sun by 1,000, then that of the penumbra would 
be about 800, and that of the umbra, 540 (Lang- 
ley). There may be 
much light and heat 
radiated by a spot, 
which seems black as 
compared with the sun ; 
for we remember that 
even a calcium light, 
held between our eyes 
and the sun, appears 
as a black spot on the 
disk of that luminary. 

Appearance of the 
sun's surface. — Even a 
telescope of moderate 
power will show the Facute. 

surface of the sun to have a peculiar mottled appear- 
3 



: '"■ ■ :■£-• 

B If! 






$&&£ 


:iwL: 


■ 




m I 


■ \ :i : y.:;^-:M±W:M,^r- : M 



50 THE SOLAR SYSTEM. 

ance not unlike that of an orange skin. But, under 
favorable circumstances and with a telescope of high 
power, the solar disk is found to be covered with 
small, intensely bright bodies irregularly distributed. 

Fig. 17. 




III 



These are now known as rice-grains.* They are 
often apparently crowded together in luminous 
ridges, or streaks, termed faculce (facula, a torch) ; 
while the rice-grains themselves, according to Prof. 
Langley, are composed of granules. Minute as a 

* Various observers describe the solar surface differently. A peculiar, elongated, 
leaf-shaped appearance of the rice-grains, called the willow-leaf structure, is shown in 
Fig. 17, as seen by Nasmyth. Newcomb compares the sun's appearance to that of a 
plate of rice-soup. Young says it frequently resembles bits of straw lying parallel to 
one another- the '• thatched -straw formation." 



PLATE n. 




THE SUN. 51 

granule seems, probably the smallest has a diameter 
of, at least, 100 miles. 

Physical Constitution of the Sun.* — Of the consti- 
tution of the sun, and the cause of the solar spots, 
very little is definitely known. 

Wilson's Theory supposed that the sun is com- 
posed of a solid, dark globe, surrounded by three 
atmospheres. The first, nearest the black body of 
the sun, is a dense, cloudy covering, possessing high 
reflecting power. The second is called the photo- 
sphere. It consists of an incandescent gas, and is 
the seat of the light and heat of the sun, being the 
sun that we see. The third, or outer one, is trans- 
parent—very like our atmosphere. 

According to this theory, the spots are to be ex- 
plained in the following manner. They are simply 
openings in these atmospheres made by powerful 
upward currents. At the bottom of these chasms, 
we see the dark sun as a nucleus at the center, and 
around this the cloudy atmosphere — the penumbra. 
This explains a black spot with its penumbra. Some- 
times the opening in the photosphere may be smaller 
than that in the inner or cloudy atmosphere ; in that 
case there will be a black spot without a penumbra. 

It will be natural to suppose that when the heated 
gas of the photosphere, or second atmosphere, is 
violently rent asunder by an eruption or current 
from below, luminous ridges will be formed by the 
heaped-up gas on every side of the opening. This 
would account for the faculce surrounding the sun- 

For the viev/s of various authorities 011 the constitution of the sun, solar spots, etc., 
see Newcomb's Astrono./iy, third edition, p. 271. 



52 



THE SOLAR SYSTEM. 



spots. It will be natural, also, to suppose that some- 
times the cloudy atmosphere below will close up first 
over the dark surface of the sun, leaving only an 
opening through the photosphere, disclosing at the 
bottom a grayish surface of penumbra. We can 

Fig. 19. 




Wilson's Theory. 



readily see, also, how, as the sun revolving on its 
axis brings a spot nearer and nearer to the center, 
thus giving us a more direct view of the opening, we 
can see more and more of the dark body. Then as 
it passes by the center the nucleus will disappear, 



THE SUN. 53 

until finally we can see only the side of the fissure, 
the penumbra, which, in its turn, will vanish. 

The Present Theory* is deduced from the re- 
sults of Spectrum Analysis, of which we shall here- 
after speak. It is constantly being modified by new 
discoveries. But we may, in general, believe the 
sun to be a vast, fiery body, surrounded by an 
atmosphere of substances volatilized by the intense 
heat. Among these, we recognize familiar elements, 
as iron, copper, &c. 

The different portions of the sun are thought to be 
arranged thus : (1). The nucleus, probably gaseous ; \ 
(2). The photosphere, an envelope several thousand 
miles thick, which constitutes the visible part of the 
sun ; (3). The chromosphere, composed of luminous 
gas, mostly hydrogen, and the seat of enormous pro- 
tuberances, tongues of fire, which dart forth, some- 
times at the rate of 150 miles per second, and to a 
distance of over 100,000 miles ; (4). The corona,% an 
outer appendage of faint, pearly light, consisting of 
streamers reaching out often several hundred thou- 
sand miles. Of these solar constituents, the eye and 
the telescope ordinarily reveal only the photosphere ; 
the rest are seen during a total eclipse or by means 
of the spectroscope. 

The outer portion of the sun radiates its heat and 

* As Kirchhoff, by his discoveries in Spectrum Analysis, laid the foundation of this 
theory, it is often called after him. 

t The interior of the sun, if gaseous, must be powerfully condensed, because of the 
tremendous pressure of the atmosphere. The high temperature, however, prevents the 
gas from liquefying. The rain-storms on the sun, if such ever occur, consist of drops 
of molten iron, copper, zinc, &c, vaporized by the enormous heat ; and often a tempest 
would drive before it this white-hot, metallic blast, with a speed of 100 miles per second. 

t This is so called because, during a total eclipse, it forms around the moon a corona, 
or glory, that is the most wonderful feature of this rare event. (See p. 141.) 



54 THE SOLAR SYSTEM. 

light, and, becoming cooler, sinks ; the hotter matter 
in the interior then rises to take its place, and thus 
convection currents are established (Physics, p. 193). 
The cooler, descending currents are darker, and the 
hotter, ascending ones are lighter ; this gives rise to 
the mottled look of the sun. At times, this occurs on 
a grand scale, and the heated, up-rushing masses 
form the f aculse, and the cooler, down-rushing ones 
produce the solar spots. 

The Heat of the Sun is generally considered to be 
produced by condensation, whereby the size of the 
sun is constantly decreasing, and its potential energy 
thus converted into kinetic. The dynamic theory 
accounts for the heat and the solar spots by assum- 
ing that there are vast numbers of meteors revolving 
around the sun, and that these constantly rain down 
upon the surface of that luminary. * Their motion, 
thus stopped, is changed to heat, and feeds this great 
central fire. Were Mercury to strike the sun in this 
way, it would generate sufficient heat to compensate 
the loss by radiation for seven years. 

Doubtless, the solar heat is gradually diminishing, 
and will ultimately be exhausted. In time, the sun 
will cease to shine, as the earth did long since. New- 
comb says that in 5,000,000 years, at the present 
rate, the sun will have shrunk to half its present 
size, and that it cannot sustain life on the earth 
more than 10,000,000 years longer. Of this we may 
be assured, there is enough to support life on our 
globe for millions of years yet to come. 

* The heat of the sun could be maintained by an annual contraction of 220 feet in 
its diameter, a decrease so insignificant as to be imperceptible with the best instru- 
ments ; or by the annual impact of meteors equal in amount to j the mass of Mercury. 



THE PLANETS. 55 



II.— THE PLANETS. 

INTRODUCTION. 

The Planets will be described in regular order, 
passing outward from the sun. In this journey, we 
shall examine each planet in turn, noticing its dis- 
tance, size, length of year, duration of day and 
night, temperature, climate, number of moons, and 
other interesting facts, showing how much we can 
know of its world-life in spite of its wonderful dis- 
tance. We shall encounter the earth in our imag- 
inary wanderings through space, and shall explain 
many celestial phenomena already partially familiar 
to us. 

In all these worlds, we shall find traces of the 
same Divine hand, molding and directing in con- 
formity to one universal plan. We shall discover 
that the laws of light and heat are invariable, and 
that the force of gravity, which causes a stone to fall 
to the ground, acts similarly upon the most distant 
planet. Even the elements of which the planets are 
composed will be familiar to us, so that a book of 
natural science published here might, in its general 
features, answer for use in a school on Mars or 
Jupiter. 

Common Characteristics {Hind). — 1. The planets 
move in the same direction around the sun : their 



56 THE SOLAR SYSTEM. 

course, as viewed from the north side of the ecliptic, 
being contrary to the motion of the hands of a 
watch. 

2. They describe elliptical paths around the sun, — 
not differing much from circles. 

3. Their orbits are more or less inclined to the 
ecliptic, and intersect it in two points — the nodes, — 
one-half of the orbit lying north, and the other south 
of the earth's path. 

4. They are opaque bodies, and shine by reflecting 
the light they receive from the sun. 

5. They rotate upon their axes in the same way 
as the earth. This we know by telescopic observa- 
tion to be the case with many planets, and by anal- 
ogy the rule may be extended to all. Hence, they 
have the alternation of day and night. 

6. Agreeably to the principles of gravitation, their 
velocity is greatest at that part of their orbit nearest 
the sun, and least at that part most distant from it ; 
in other words, they move quickest in perihelion, and 
slowest in aphelion. 

Comparison of the two Groups of the Major 
Planets. (Chambers.) — Separating the major planets 
into two groups, if we take Mercury, Yenus, the 
Earth, and Mars as belonging to the interior, and 
Jupiter, Saturn, Uranus, and Neptune to the exterior 
group, we shall find that they differ in the following 
respects : 

1. The interior planets, with the exception of the 
Earth and Mars, are not attended by any satellite, 
while all the exterior planets have satellites. 

2. The average density of the first group consider- 



THE PLANETS. 



5? 



ably exceeds that of the second, the approximate 
ratio being 5:1. 

3. The mean duration of the axial rotations, or the 
mean length of the day of the interior planets, is 
much longer than that of the exterior ; the average 
in the former case being about twenty-four hours, 
but in the latter only about ten hours. 

Properties of the Ellipse. — In Fig. 20, S and S' are 
the foci of the ellipse ; A C is the major axis ; B D, 
the minor or conjugate axis; O, the center: or, 
astronomically, O A is the semi-axis-major or mean 




An Ellipse. 



distance, O B the semi-axis-minor : the ratio of O S 
to O A is the- eccentricity ; the least distance, S A, is 
the perihelion distance ; the greatest distance, S C, 
the aphelion distance. 

Characteristics of a Planetary Orbit. — It will not 
be difficult to follow in the mind the additional 
characteristics of a planet's orbit. Take two hoops, 
and bind them into an oval shape. Incline one 



58 The: solar system. 

slightly to the other, as shown in Fig. 21. Let the 
horizontal hoop represent the ecliptic. Imagine a 
planet following the inclined hoop, or ellipse ; at a 
certain point it rises above the level of the ecliptic : * 
this point is called the ascending node, and the op- 

Fig. 21. 




Planetary Orbits. 

posite point of intersection is termed the descending 
node. A line connecting the two nodes is the line of 
the nodes. The longitude of the node is its distance 
from the first point of Aries, measured on the eclip- 
tic, eastward. 

Comparative Size of Planets (Chambers). — The following scheme 
will assist in obtaining some notion of the magnitude of the planetary 
s}^stem. Choose a level field or common ; on it place a globe two feet 
in diameter for the Sun : Vulcan will then be represented by a small 
pin's head, at a distance of about twenty-seven feet from the center 
of the ideal sun ; Mercury by a mustard-seed, at a distance of eighty-two 
feet ; Venus by a pea, at a distance of 142 feet ; the Earth, also, by a pea, 
at a distance of 215 feet ; Mars by a small pepper-corn, at a distance of 
327 feet ; the minor planets by grains of sand, at distances varying from 
500 to 600 feet. If space will permit, we may place a moderate-sized 



* Lockyer beautifully says : " We may imagine the earth floating around the sun on 
a boundless ocean, both sun and earth being half immersed in it. This level, this plane, 
the plane of the ecliptic (because all eclipses occur in it), is used by astronomers as we 
use the sea-level. We say a mountain is so far above the level of the sea. The astrono- 
mer says the star is so high above the level of the ecliptic. 



THE PLANETS. 



59 



orange nearly one-quarter of a mile distant from the starting point to rep- 
resent Jupiter ; a small orange two-fifths of a mile for Saturn ; a full-sized 
cherry three-quarters of a mile distant for Uranus ; and lastly, a plum 
l\ miles off for Neptune, the most distant planet yet known. Extending 




Comparative Size of the Planets. 



this scheme, we should find that the aphelion distance of Encke's comet 
would be at 880 feet ; the aphelion distance of Donati's comet of 1858 at 
six miles ; and the nearest fixed star at 7,500 miles. 



60 



THE SOLAR SYSTEM. 



According to this scale, the daily motion of Vulcan in its orbit would be 
4| feet ; of Mercury, 3 feet ; of Venus, 2 feet ; of the Earth, If feet ; of 
Mars, 1-| feet ; of Jupiter, 10| inches ; of Saturn, 1\ inches ; of Uranus, 5 
inches ; and of Neptune, 4 inches. This illustrates the fact that the orbital 
velocity of a planet decreases as its distance from the sun increases. * 

Conjunction of Planets. — The grouping together 
of two or more planets within a limited area of the 
heavens is a rare event. The earliest record we 
have is the one of Chinese origin (p. 6), stating that 
a conjunction of Mars, Jupiter, Saturn, and Mercury 




Venus and Jupiter in Conjunction, January 30, 1868. 

occurred in the reign of the Emperor Chuenhio. 
Astronomers tell us that this took place Feb. 28, 2446 
b. c, between 10° and 18° of Pisces. There is a very 
general impression, however, that this conjunction 
was afterward calculated and chronicled in their 
records. In 1725, Venus, Mercury, Jupiter, and 

* If we accept the Nebular Hypothesis (p. 255), we can easily understand the reason 
of this ; the exterior planets, being made earlier, had the motion of the nebula during 
-its earlier stage. The rotation-velocity of the nebula kept increasing, and so, of course, 
each planet possessed a higher rate of orbital speed than the preceding one. 



THE PLANETS. 61 

Mars appeared in the same field of the telescope. In 
1859, Venus and Jupiter came so near each other 
that they appeared to the naked eye as one object. 

Are the Planets Inhabited? — This question is one 
which very naturally arises, when we think of the 
planets as worlds in so many respects similar to our 
own. We can give no satisfactory answer. Many 
think that the only object God can have in making 
a world is to form an abode for man. Our own earth 
was evidently fitted up, although perhaps not cre- 
ated, for this express purpose. Everywhere about 
us we find proofs of special forethought and 
adaptation. Coal and oil in the earth for fuel and 
light, forests for timber, metals in the mountains 
for machinery, rivers for navigation, and level 
plains for corn. The human body, the air, light, and 
heat are all fitted to one another with exquisite 
nicety. 

When we turn to the planets, we do not know but 
God has other races of intelligent beings who inhabit 
them, or even entirely different ends to attain. Of 
this, however, we are assured, that, if inhabited, the 
conditions on which life is supported vary much 
from those familiar to us. When we come to speak 
of the different planets, we shall see- (1) how they 
differ in light and heat, from seven times our usual 
temperature to less than -j-^ ; (2) in the intensity 
of the force of gravity, from 2-J- times that of 
the earth to less than 4 ; (3) in the constitution of 
the planet itself, from a density \ heavier than that 
of the earth to one nearly that of cork. 

The temperature may often sweep downward 



62 



THE SOLAR SYSTEM. 



through a scale of 2,000° in passing from Mercury 
to Neptune. No human being could reside on the 
former, while we cannot conceive of any polar inhab- 




Size of the Sun as seen from the Planets. 



itant who could endure the intense cold of the latter. 
At the sun, one of our pounds would weigh over 27 
pounds ; on our moon, the pound weight would be- 



THE PLANETS. 63 

come only about two ounces ; while on Vesta, one of 
the planetoids, a man could easily spring sixty feet 
in the air and sustain no shock in falling. Yet, while 
we speak of these peculiarities, we do not know 
what modification of the atmosphere or physical 
features may exist on Mercury to temper the heat, 
or on Neptune to reduce the cold. 

With all these diversities, we must, however, admit 
the power of an all-wise Creator to form beings 
adapted to the life and the land, however different 
from our own. The Power that prepared a world 
for us, could as easily and perfectly prepare one for 
other races. May it not be that the same love of 
diversity, that will not make two leaves after the 
same pattern nor two pebbles of the same size, de- 
lights in worlds peopled by races as diverse ? * 

While, then, we cannot affirm that the planets are 
inhabited, analogy would lead us to think that they 
are, and that the most distant star that shines in 
the arch of heaven may give light and heat to living 
beings under the care and government of Him who 
enlivens the densest forest with the hum of insects, 
and populates even a drop of water with its teeming 
millions of animalcules. 

Divisions of the Planets. — The planets are divided 
into two classes : (1). Inferior, or those whose orbits 
are within that of the earth — viz,, Mercury, Venus ; 
(2). Superior, or those whose orbits are beyond that 

* Astronomers conceive the universe to contain worlds in every possible stage of 
development, from the primary, gaseous nebula, to a worn-out, dead globe, like the 
moon. At a certain period in its existence, each world may be fitted to support life. 
Millions may now be in that condition ; others may be approaching, while others have 
passed it. 



64 TIfE SOLAR SYSTEM. 

of the earth — viz., Mars, Jupiter, Saturn, Uranus, 
Neptune. 

Motions of a Planet as seen from the Sun. — Could 
we stand at the sun and watch the movements of the 
planets, they would all be seen revolving with dif- 
ferent velocities in the order of the zodiacal signs. 
But to us, standing on one of the planets, itself in 
motion, the effect is changed. To an observer at the 
sun all the motions would be real, while to us many 
are only apparent. The position of a planet, as seen 
from the center of the sun, is called its heliocentric 
place ; as seen from the center of the earth, its geo- 
centric place. When Venus is at inferior conjunc- 
tion, an observer at the sun would see it in the oppo- 
site part of the heavens from that in which it would 
appear to him if viewed from the earth. 

Motions of an Inferior Planet — An inferior planet 
is never seen by us in any part of the sky opposite 
to the sun at the time of observation. It cannot 
recede from him as much as 90°, or \ the circum- 
ference, since it moves in an orbit entirely enclosed 
by the orbit of the earth. Twice in every revolution 
it is in conjunction ( 6 ) with the sun, — an inferior 
conjunction (A) when it comes between the earth 
and the sun, and a superior conjunction (B) when 
the sun lies between it and the earth. 

When the planet attains its greatest distance east 
or west (as we see it) from the sun, it is said to be at 
its greatest elongation. 

When passing from B to A it is east of the sun, 
and from A to B it is west of the sun. When east of 
the sun, it sets later than the sun, and hence is 






THE PLANETS. 



65 



evening star : when west of the sun, it rises earlier 
than the sun, and hence is morning star. An inferior 
planet is never visible when in superior conjunction, 
as its light is then lost in the greater brilliancy of 
the sun. When in inferior conjunction, it some- 

Fig. 25 




Conjunctions of Inferior Planet. 



times passes in front of the sun, and appears to us as 
a round, black spot swiftly moving across his disk. 
This is called a transit. 

Retrograde Motion of an Inferior Planet. — 
Suppose the earth at A (Fig. 26), and the planet at B. 



66 



THE SOLAR SYSTEM. 



Now, while the earth is passing to F, the planet will 
pass to D,— the arc AF being shorter than BD, be- 
cause the nearer a planet is to the sun the greater its 
velocity. While the planet is at B, we locate it at 
C on the ecliptic, in Gemini ; but at D, it appears to 
us to be at G, in Taurus. So that the planet has 




Retrograde Motion. 



retrograded through an entire sign on the ecliptic, 
while its course all the while has been directly for- 
ward in the order of the signs ; and to an observer at 
the sun, such would have been its motion. 

Phases of an Inferior Planet. — An inferior 
planet presents all the phases of the moon. At supe- 
rior conjunction, the whole illumined disk is turned 
toward us ; but the planet is lost in the sun's rays : 



THE PLANETS. 67 

therefore neither Mercury nor Venus ever presents a 
complete circular appearance, like the full moon. A 
little before or after superior conjunction, an inferior 
planet may be seen with a telescope ; but the whole 
of the light side is not turned toward us, and so the 
planet appears gibbous, like the moon between the 
first quarter and full. At its greatest elongation, 
the planet shows us only one-half its illumined disk ; 
this decreases, becoming more and more crescent 
toward inferior conjunction, at which time the un- 
illumined side is toward us. 

Fig. 27. 

if) © o* 

Phases of an Inferior Planet. 

Motions of a Superior Planet. — The superior planet 
moves in an orbit which entirely surrounds that of the 
earth. When the earth is at E (Fig. 28), the planet 
at L is said to be in opposition to the sun (8). It 
is then at its greatest distance from him — 180°. The 
planet is on the meridian at midnight, while the sun 
is on the corresponding meridian on the opposite side 
of the earth ; or the planet may be rising, when the 
sun is just setting. When the planet is at N, it is in 



68 



THE SOLAR SYSTEM. 



conjunction, and being lost in the sun's rays is invis- 
ible to us. When 90° east or west of the sun, the 
planet is said to be in quadrature (d). 

Ketrograde Motion of a Superior Planet. — Sup- 
pose the earth to be at E and the planet at L, and 
that we move on to G while the planet passes on to 




T K 



S3 

Retrograde Motion of a Superior Planet. 

— the distance EG being longer than LO, the re- 
verse of what takes place in the movements of the 
inferior planets ; at E, we should locate the planet at 
P on the ecliptic, in the sign Cancer ; but at G, it 
would appear to us at Q, in the sign Gemini, having 



THE PLANETS. 69 

apparently retrograded on the ecliptic the distance 
PQ, while it was all the time moving on in the 
direct order of the signs. Now, suppose the earth 
passes on to I and the planet to U, we should then 
see it at the point W, further on in the ecliptic than 
Q, which indicates direct motion again, and at some 
point near Q the planet must have appeared without 
motion. 

After this, it will continue direct until the earth has 
completed a large portion of her orbit, as we can 
easily see by imagining various positions of the earth 
and planet, and then drawing lines as we have just 
done, noticing whether they indicate direct or retro- 
grade motion. The greater the distance of a planet 
the less it will retrograde, as we can perceive by 
drawing another orbit outside the one represented in 
the cut, and making the same suppositions concern- 
ing it as those we have already explained. 

Sidereal and Synodic Revolution. — The interval 
of time required by a planet to perform a revolution 
from one fixed star back to it again, is termed a 
sidereal revolution (siclus, a star). 

1. The interval of time between two similar con- 
junctions of an inferior planet with the earth and 
the sun is termed a synodic revolution. Were the 
earth at rest, there would be no difference between a 
sidereal and a synodic revolution, and the planet 
would come into conjunction twice in each revolu- 
tion. Since, however, the earth is in motion, it fol- 
lows that, after the planet has completed its sidereal 
revolution, it must overtake the earth before they 
can both come again into the same position with 



70 



THE SOLAR SYSTEM. 



regard to the sun. The faster a planet moves, the 
sooner it can do this. Mercury, traveling at a 
greater speed and on an inner orbit, accomplishes 
it much more quickly than Venus. The synodic 
period always exceeds the sidereal. 

2. The interval between two successive conjunc- 
tions or oppositions of a superior planet is also 
termed a synodic revolution. Since the earth moves 
so much faster than any superior planet, it fol- 
lows that, after it has completed a sidereal revo- 
lution, it must overtake the planet before they can 
again come into the same position with regard to 
the sun. The slower the planet, the sooner this 
can be done. Uranus, making a sidereal revolution 
in eighty-four years, can be overtaken more quickly 
than Mars, which makes one in less than two 
years. It consequently requires over a second 
revolution for the earth to catch up with Mars, 
only ^ of a second one to overtake Jupiter, and 
but little over -^ of a second one to come up with 
Uranus. 

Planets as Evening and Morning Stars. — The in- 
ferior planets are evening stars from superior to 
inferior conjunction ; and the superior planets, from 
opposition to conjunction. During the other part of 
their revolutions, they are morning stars. 



13 



Mercury is evening star about 2 months. 

Venus 

Mars 

Jupiter 

Saturn 

XJranus 



THE PLANETS. 71 



I. VULCAN (hypothetical). 

Supposed Discovery. — Le Verrier, having detected an error in the 
assumed motion of Mercury, suggested, in the autumn of 1859, that there 
might be an interior planet, which was the cause of this disturbance. On 
this being made public, M. Lescarbault, a French physician and an 
amateur astronomer, stated that on March 26 of that year he had seen a 
dark body pass across the sun's disk, which might have been the 
unknown planet. Le Verrier visited him, and found his instruments 
rough and home-made, but singularly accurate. His clock was a simple 
pendulum, consisting of an ivory ball hanging from a nail by a silk thread. 
His observations were on prescription paper, covered with grease and 
laudanum. His calculations were chalked on a board, which he planed off 
to make room for fresh ones. Le Verrier became satisfied that a new planet 
had been discovered by this enthusiastic observer, and congratulated him 
upon his deserved success. 

On March 20, 1862, Mr. Lummis, of Manchester, England, noticed a 
rapidly-moving, dark spot, apparently the transit of an inner planet. 
During the total eclipse of July 29, 1878, Professor Watson, of Ann Arbor 
Observatory, and Dr. Lewis Swift, of Rochester, claimed to have seen 
two Intra-Mercurial planets. As yet, however, the existence of the planet 
is not generally conceded. The name Vulcan and the sign of a hammer 
have been given to it. Its distance from the sun has been estimated at 
13,000,000 miles, and its periodic time (its year) at twenty days. 

II. MERCURY. 

The fleetest of the gods. Sign, $ , his wand. 

Description. — Mercury is nearest to the sun of any 
of the definitely-known planets. When the sky is 
very clear, we may sometimes see it, just after sun- 
set, as a bright, sparkling star, near the western 
horizon. Its elevation increases evening by evening, 



72 THE SOLAR SYSTEM. 

but never exceeds 28°.* If we watch it closely, we 
shall find that the planet again approaches the sun 
and becomes lost in his rays. Some days after- 
ward, just before sunrise, we can see the same planet 
in the east, rising higher each morning, until its 
greatest elevation equals that which it before at- 
tained in the west. Thus the planet appears slowly 
but steadily to oscillate like a pendulum, to and fro, 
from one side to the other of the sun. The ancients, 
deceived by this puzzling movement, failed to dis- 
cover the identity of the two stars, and called the 
morning star Apollo, the god of day, and the evening 
star Mercury, the god of thieves, who walk to and 
fro in the night-time seeking plunder. \ 

On account of the nearness of Mercury to the sun, 
it is difficult to be detected. \ It is said that Coper- 
nicus, an old man of seventy, lamented in his last 
moments that, much as he had tried, he had never 
been able to see it. In our latitude and climate, we 
can generally easily find it if we watch for it at the 
time of its greatest elongation, as commonly given 
in the almanac. 

Motion in Space. — Mercury revolves around the 
sun at a mean distance of about 36,000,000 miles. Its 

* This distance varies much, owing to the -eccentricity of Mercury's orbit. 

t The Greeks gave to Mercury the additional name of "The Sparkling One." The 
astrologists looked upon it as the malignant planet. The chemists, because of its 
extreme swiftness, applied the name to quicksilver. The most ancient account that we 
have of this planet is given by Ptolemy, in his Almagest ; he states 'its location on the 
15th of November, 265 b. c. The Chinese also state that on June 9, 118 a. r>., it was 
near the Beehive, a cluster of stars in Cancer. Astronomers tell us that, according to 
the best calculations, it was at that date within less than 1° of that group. 

X An old English writer by the name of Goad, in 1686, humorously termed this planet, 
" A squinting lacquey of the sun, who seldom shows his head in these parts, as if he 
were in debt," 



MERCURY. 73 

orbit is the most eccentric (flattened) of any among 
the eight principal planets, so that, although when in 
perihelion it approaches to within about 28,000,000 
miles, in aphelion it speeds away 15,000,000 miles 
further, or to the distance of over 43,000,000 miles. 
Being so near the sun, its motion in its orbit is cor- 
respondingly rapid, — viz., thirty miles per second.* 

The Mercurial year comprises only about eighty- 
eight days, or nearly three of our months. Mercury 
is thought to rotate upon its axis in about the same 
time as the earth, so that the length of the Mercurial 
day is nearly the same as that of the terrestrial 
one. 

Though Mercury thus completes a sidereal revolu- 
tion around the sun in eighty-eight days, yet to pass 
from one inferior or superior conjunction to the next 
(a synodic revolution) requires 116 days. The reason 
of this is, that when Mercury comes around again 
to the point of its last conjunction, the earth has 
gone forward, and it requires twenty-eight days for 
the planet to overtake us. 

The Distance from the Earth varies still more 
than the distance from the sun. At inferior con- 
junction, Mercury is between the earth and the sun, 
and its distance from us is the difference between 
the distance of the earth and of the planet from the 
sun : at superior conjunction, it is the sum of these 
distances. Its apparent diameter in these different 
positions varies in the same proportion as the dis- 
tance, or nearly three to one. The greatest and least 

* At this rate of speed, we could cross the Atlantic Ocean in two minutes. 

4 



74 THE SOLAR SYSTEM. 

distances vary as either planet happens to be in 
aphelion or perihelion.* 

Dimensions. — Mercury is about 3,000 miles in di- 
ameter. Its volume is about fa that of the earth — 
i. e., it would require twenty globes as large as Mer- 
cury to make one the size of the earth, or 25,000,000 
to equal the sun. It is | denser than the earth, its 
mass is nearly -fa that of the earth, and a stone let 
drop upon its surface would fall 7 J feet the first 
second. Its specific gravity is not far from that of 
tin. A pound weight removed to Mercury would 
weigh only about seven ounces. 

Seasons. — As Mercury's axis is much inclined from 
a perpendicular (perhaps 70°), its seasons are peculiar. 
There are no distinct frigid zones ; but large regions 
near the poles have six weeks of continuous day and 
torrid heat, alternating with a night of equal length 
and arctic cold. The sun shines perpendicularly 
upon the torrid zone only at the equinoxes, while he 
sinks far toward the southern horizon at one solstice, 
and as far toward the northern horizon at the 
other, f The equatorial regions, therefore, during 
each revolution, are modified in their temperature 
from torrid to temperate, and the tropical heat is 
experienced alternately toward the north and the 
south of what we call the temperate zones. 

There is no marked distinction of zones as with us, 
but each zone changes its character twice during the 

* If at inferior conjunction Mercury is in aphelion and the earth in perihelion, its 
distance from us is only 91,500,000—43,000,000 = 48,500,000 miles. If at superior con- 
junction Mercury is in aphelion and the earth in aphelion also, its distance from us 
is 94,500,000 + 43,000,000 = 137,500,000 miles. 

t Read a chapter entitled " The Fiery World," in Proctor's Poetry of Astronomy. 



MERCURY. 75 

Mercurial year, or eight times during the terrestrial 
one. An inhabitant of Mercury must be accustomed 
to sudden and violent vicissitudes of temperature. 
At one time, the sun not only thus pours down its 
vertical rays, and in a few weeks after sinks far 
toward the horizon, but, on account of Mercury's 

Fig. 29. 




Orbit and Seasons of Mercury. 

elliptical orbit, when in perihelion the planet ap- 
proaches so near the sun that the heat and light are 
ten times as great as ours, while in aphelion it re- 
cedes so as to reduce the amount to four and a half 
times. The average heat is about seven times that 
of the earth, — a temperature sufficient to turn water 
into steam, and even to melt zinc. 



76 THE SOLAR SYSTEM. 

The relative length of the days and nights is much 
more variable than with us. /The sun, apparently 
seven times as* large as it. seems to us, must be a 
magnificent ■■spectacle, cand:; illumine .every object 
with insufferable brilliancy^; The evening sky is, 
however, lighted by no moon. _" ' /. c :' J s L: . . • . 

Telescopic Features. — Through the telescope, Mer- 
cury presents all the phases of the moon, from a slen- 
der crescent to gibbous, after which its light is lost in 
that of the sun. These phases prove that Mercury is 
spherical, and shines by the light reflected from the 
sun. Being an inferior planet, we never see it when 
full, and hence the brightest, nor when nearest the 
earth, as then its dark side is turned toward us. 

Owing to the dazzling light, and the vapors almost 
always hanging around our horizon, this planet has 
not of late received much attention ; the data here 
given are mainly based upon the observations of the 
older astronomers, and are, therefore, not universally 
accepted. Mercury is thought by some to have a 
dense, cloudy atmosphere, that materially dimin- 
ishes the intensity of its heat and, perhaps, makes 
it habitable, though others assert that the atmos- 
phere is too insignificant to be detected. Some dark 
bands about the planet's equator indicate, perhaps, 
an equatorial zone. There are, also, lofty heights 
which intercept the light of the sun, and deep valleys 
plunged in shade. One mountain is claimed to be 
over eleven miles high, or about ^ the diameter of 
the planet.* 

* The height of the loftiest peak of the Himalayas is only 29,000 feet, or about j^b 
part of the earth's diameter. 



VENUS. 11 



III. VENUS. 

The Queen of Beauty. Sign ? , a looking-glass. 

Description. — Venus, the next in order to Mercury, 
is the most brilliant of the planets.-* She presents 
the same appearances as Mercury. w Gwiog; however, 
to the larger size of her orbit, her greatest apparent 
oscillations are nearly 48° east and west of the~s/un,t 
or about 20° more than those of Mercury. She is 
therefore seen much earlier in the morning and 
much later at night. She is morning star from in- 
ferior to superior conjunction, and evening star from 
superior to inferior conjunction. 

Venus is the most brilliant about five weeks before 
and after inferior conjunction, at which time the 
planet is bright enough to cast a shadow at night. 
If, in addition, at this time of greatest brilliancy, 
Venus is at or near her highest north latitude, she 
may be seen with the naked eye in full daylight. X 
This occurs once in eight years — the interval required 
for the earth and planet to return to the same situa- 
tion in their orbits ; eight complete revolutions of the 

* When visible before sunrise, she was called by the ancients Phosphorus, Lucifer, 
or the Morning Star, and when she shone in the evening after sunset, Hesperus, Vesper, 
or the Evening Star. 

t This distance varies only about 3°, owing to the slight eccentricity of Venus's 
orbit. 

\ Arago relates that Buonaparte, upon repairing to the Luxembourg, when the 
Directory was about to give him a fete, was much surprised at seeing the multitude 
paying more attention to the heavens above the palace than to him or his brilliant staff. 
Upon inquiry, he learned that these curious persons were observing with astonishment 
a star which they supposed to be that of the Conqueror of Italy. The emperor himself 
was not indifferent when his piercing eye caught the clear lustre of Venus smiling upon 
him at midday. 



78 THE SOLAR SYSTEM. 

earth about the sun occupying nearly the same time 
as thirteen of Venus. 

Motion in Space. — Venus has an orbit the most 
nearly circular of any of the principal planets. Her 
mean distance from the sun is about 67,000,000 miles, 
which varies at aphelion and perihelion 1,000,000 
miles, — a contrast to Mercury, which varies 15,000,000 
miles. 

Venus makes a complete revolution around the 
sun in about 225 days, at the mean rate of twenty- 
two miles per second ; hence her year is equal to 
about seven and one-half of our months. This is a 
sidereal revolution, as it would appear to an ob- 
server at the sun ; a synodic revolution requires 584 
days. 

Mercury, we remember, catches up with the earth 
in twenty-eight days after it reaches the point where 
it left the earth at the last inferior conjunction. But 
it takes Venus nearly two and a half revolutions to 
overtake the earth and come into the same conjunc- 
tion again. This grows out of the fact that she 
has a longer orbit than Mercury, and moves only 
about one-sixth faster than the earth, while Mer- 
cury travels nearly twice as fast as our planet. 
Venus rotates upon her axis in about twenty-four 
hours ; so the length of her day does not differ essen- 
tially from ours. 

Distance from the Earth. — Like that of Mercury, 
the distance of Venus from the earth, when in in- 
ferior conjunction, is the difference between the dis- 
tances of the two planets from the sun ; when 
in superior conjunction, the sum of these distances. 



VENUS. 79 

When nearest to us, Venus is only about 25,000,000 
miles away. 

Figure 30 represents her apparent dimensions at 
the extreme, mean, and least distances from us. 
The variation is nearly as the numbers 10, 18, and 65. 
It would be natural to think that the planet is the 
brightest when the nearest, and thus the largest, but 

Fig. 30. 




Extreme, Mean, and Least Apparent Size of Venus ; and her Phases. 

we should remember that then the bright side is 
toward the sun, and the unillumined side toward us. 
Indeed, at the period of greatest brilliancy, of which 
we have spoken, only about one-fourth of her light is 
visible. At this time, however, observers have 
noticed the entire contour of the planet to be of a 
dull gray hue, as seen in the cut. 

Dimensions.— Venus is about 7,600 miles in diame- 
ter. The volume and density of the planet are each 
about nine-tenths that of the earth. A stone let fall 
upon her surface would fall fourteen feet in the first 



$0 THE SOLAR SYSTEM. 

second : a pound weight removed to her equator 
would weigh about fourteen ounces. From this we 
see that the force of gravity does not decrease 
exactly in proportion to the size of the planet, any 
more than it increases with the size of the sun. 
The reason is, that the body is brought nearer the 
mas& of the^ small planet, and so feels its attraction 
more fully than when far out upon the circumfer- 
ence of a large body, — the attraction increasing as 
the square of the distance from the particles de- 
creases. 

Seasons. — Since the axis of Venus is very much 
inclined from a perpendicular, her seasons are similar 

Fig. 31. 




at her Solstice. 



to those of Mercury. The torrid and temperate zones 
overlap each other, and the polar regions have, alter- 
nately, at one solstice a torrid temperature, and at 
the other a prolonged arctic cold. The inequality of 
the nights is very marked. The heat and light are 



VENUS. 81 

double that of the earth, while the circular form of 
her orbit gives nearly an equal length to her four 
seasons. 

If the inclination of her axis is 75°, as some as- 
tronomers hold, her tropics must be 75° from the 
equator, and her polar circles 75° from the poles. The 
torrid zone is, therefore, 150° in width. The torrid 
and frigid zones interlap through a space of 60°, mid- 
way between the equator and the poles. 

Telescopic Features. — Venus, being an interior 
planet, presents, like Mercury, all the phases of the 
moon.* 

She is thought to have a dense, cloudy atmosphere. 
This was suggested by the fact that at the transit of 

Fig. 32. 




Crescent and Spots of Venus. 

Venus Over the sun in 1761, 1769, and 1882, a faint 
ring of light surrounded the black disk of the planet. 

* This was discovered by .Galileo, and was among the first achievements of his tele- 
scopic observations. It had been argued against the Copernican system that,- if true, 
Venus should wax and wane like the moon. Indeed, Copernicus himself boldly declared 
that, if means of seeing the planets more distinctly were ever invented, Venus would be 
found to present such phases. Galileo, with his telescope, proved this feet, and 
thus vindicated the Copernican theory. 



82 THE SOLAR SYSTEM. 

The evidence of an atmosphere, as well as of moun- 
tains, however, rests upon the peculiar appearance 
attending her crescent shape. 

1. The luminous part does not end abruptly ; on 
the contrary, the light diminishes gradually. This 
diminution can be explained by a twilight caused by 
an atmosphere which diffuses the rays of light into 
regions of the planet where the sun is already set. 
Thus, on Venus, as on the earth, the evenings are 
lighted by twilight, and the mornings by dawn. 

2. The edge of the enlightened portion of the 
planet is uneven and irregular. This appearance is 
doubtless the effect of shadows cast by mountains. 

Spots have been noticed on her disk which are con- 
sidered to be traceable to clouds. Herschel thinks 
that we never see the body of the planet, but only her 
atmosphere loaded with vapors, which may mitigate 
the glare of the intense sunshine. 

Satellites. — Venus is not known to have any 
moon. 



IV. THE EARTH. 

Sign, <$», a circle with Equator and Meridian. 

The Earth is the next planet we meet in passing 
outward from the sun. To the beginner, it seems 
strange enough to class our world among the heav- 
enly bodies. They are brilliant, while it is dark and 
opaque ; they appear light and airy, while it is solid 
and firm ; we see in it no motion, while they are 



THE EARTH. 



constantly changing their position ; they seem mere 
points in the sky, while it is vast and extended. 
Yet, at the very beginning, we are to consider the 



Fig. 33. 




The Earth in Space. 



earth as a planet shining brightly in the heavens, 
and appearing to other worlds as a planet does to us, 



84 THE SOLAR SYSTEM. 

We are to learn that" it is in motion, flying through 
its orbit with inconceivable velocity; that it is not 
fixed, but hangs in space, held by an invisible 
power of gravitation which it cannot evade ; * that 
it is small and insignificant beside the mighty globes 
that so gently shine upon us in the far-off sky ; that, 
in fact, it is only one atom in a universe of worlds, 
all firm and solid, and all, perhaps, equally fitted to 
be the abode of life. 

Dimensions. — The earth is not "round like a ball," 
but flattened at the poles. Its form is that of an 
oblate spheroid. Its polar diameter is about 7,899 
miles, and its equatorial about 7, 925 J. The com- 
pression is, therefore, 26| miles. (See table in 
Appendix.) If we represent the earth by a globe 
one yard in diameter, the polar diameter would be 
one-tenth of an inch too long. The circumfer- 
ence of the earth is nearly 25,000 miles. Its density 
is about 5 \ times that of water. Its weight is 
6,069,000,000,000,000,000,000 tons. 

The inequalities of the earth's surface, arising 
from valleys, mountains, etc. , have been likened to 
the roughness on the rind of an orange. On a globe 
sixteen inches in diameter, the land, to be in pro- 
portion, should be represented by the thinnest writing 
paper, the hills by very fine grains of sand, and ele- 
vated ranges by thick drawing-paper. To represent 
the^eepest_wells or mines, a scratch should be made 
that would be invisible except with a. glass. 

. •"* Were - the_sun's" attractive 'force upon the earth replaced by the largest steel tele- 
graph wire, it would require nine wires for each square inch of the sunward side of our 
globe", toehold the 5 earth in her orbit, I '.....-. 



THE EARTH. ■ 85 

The Rotundity of the Earth is proved in various 
ways : (1) By the fact that vessels have sailed 
around the earth ; * (2) when a ship is coming into 
port, we see the masts first ; (3) the shadow of the 
earth on the moon is circular ; (4) the polar star 
seems higher in the heavens as we pass north ; and 
(5) the horizon expands as we ascend an eminence, f 
If we climb to the top of a hill, we can see further 
than when on the plain at its foot. Our eyesight is 
not improved ; it is only because ordinarily the cur- 
vature of the earth shuts off the view of distant 
objects, but when we ascend to a higher point, we 
can see further over the side of the earth. The cur- 
vature is eight inches per mile, 2 2 x 8 in - = 32 inches 
for two miles, 3 2 x 8 in - for three miles, etc. An 
object of these respective heights would be just 
hidden at these distances. 

Apparent and Real Motion. — In endeavoring to 
understand the various appearances of the heavenly 
bodies, it is well to remember how in daily life we 



* It is curious, in connection with this well-known fact, to recall the arguments 
urged by the Spanish philosophers against the reasoning of Columbus, when he assured 
them that he could arrive at Asia just as certainly by sailing west as east. "How," 
they asked, " can the earth be round ? If it were, then on the opposite side the rain 
would fall upward, trees would grow with their branches down, and everything would 
be topsy-turvy. Every object on its surface would certainly fall off, and if a ship by 
sailing west should get around there, it would never be able to climb up the side of the 
earth and get back again. How can a ship sail up hill ? " 

t " The history of aeronautic adventure affords a curious illustration of this same 
principle. The late Mr. Sadler, the celebrated aeronaut, ascended on one occasion in a 
balloon from Dublin, and was wafted across the Irish Channel, when, on his approach 
to the Welsh coast, the balloon descended nearly to the surface of the sea. By this time 
the sun was set, and the shades of evening began to close in. He threw out nearly all 
his ballast, and suddenly sprang upward to a great height, and by so doing brought his 
horizon to dip below the sun, producing the whole phenomenon of a western sunrise. 
Subsequently descending in Wales, he, of course, witnessed a second sunset on the same 
evening." 



86 . THE SOLAR SYSTEM. 

transfer motion. On the cars, when in rapid move- 
ment, the fences and the trees seem to glide by us, 
while we sit still and see them pass. On a bridge, 
when we are at rest, we watch the undulations of 
the waves, until at last we come to think that they 
are stationary and we are sweeping up the stream*. 

' ' In the cabin of a large vessel going smoothly before the wind on still 
Avater, or drawn along a canal, not the smallest indication acquaints us 
with the ' way it is making. ' We read,, sit, walk, as if we were on land. 
If we throw a ball into the air, it falls back into our hand ; if we drop it, 
it alights at our feet. Insects buzz around us as in the free air, and smoke 
ascends in the same manner as it would do in an apartment on shore. If, 
indeed, we come on deck, the case is in some respects different ; the air, 
not being carried along with us, drifts away smoke and other light bodies, 
such as feathers cast upon it, apparently in the opposite direction to that 
of the ship's progress ; but in reality they remain at rest, and we leave 
them behind in the air. And what is the earth itself but the good ship we 
are sailing in through the universe, bound round the sun ; and as we sit 
here in one of the ' berths,' we are unconscious of there being any ' way ' 
at all upon the vessel. On deck, too, out in the open air, it's all the same 
so long as we keep our eyes on the ship ; but immediately we look over 
the sides — and the horizon is but the ' gunwale ' of our vessel — we see the 
blue tide of the great ocean around us go drifting by the ship, and spark- 
ling with its million stars as the waters of the sea itself sparkle at night 
between the tropics. " 

Diurnal Rotation of the Earth around its Axis. 

— The earth, in constantly turning from west to east, 
elevates our horizon above the stars on the west, and 
depresses it below the stars on the east. As the 
horizon appears to us to be stationary, we assign the 
motion to the stars, thinking those on the west, 
which it passes over and hides, to have sunk below it, 
or set ; and imagining those on the east, below which 



THE EARTH. 



87 



it has dropped, to have moved above it, or risen. So, 
also, the horizon is depressed below the sun, and we 
call it sunrise ; it is elevated above the sun, and we 
call it sunset. 

We thus see that the diurnal movement of the sun 
by day and the stars by night is an optical illusion, 
— that here as elsewhere we simply transfer motion. 
This seems easy enough for us to understand ; but it 
was the "stone of stumbling" to ancient astrono- 
mers for thousands of years. Copernicus himself, it 
is said, first thought of the true solution while riding 
on a vessel and noticing how he insensibly trans- 
ferred the movement of the ship to the objects on 
the shore. How much grander the beautiful sim- 
plicity of this system than the cumbersome com- 
plexity of the old Ptolemaic belief ! 

Diurnal Motion of the Sun. — The explanation 
just given illustrates the apparent motion of the sun, 
and the cause of day and night. Suppose S to be the 
sun. The earth, E, turning upon its axis EF from 

Fig. 3/f. 




Daily Motion of the Sun (Hind). 




west to east, has only half its surface illuminated at 
one time by the sun. To a person at D, the sun is in 
the horizon and day commences; that luminary ap- 



88 THE SOLAR SYSTEM. 

pearing to rise higher and higher, with a westerly 
motion, as the observer is carried forward easterly 
by the earth's diurnal rotation to A, where he has 
the sun in his meridian, and it is consequently noon. 
The sun then begins to decline in the sky until the 
spectator arrives at B, where it sets, or is again in 
the horizon on the west side, and night begins. He 
moves on to C, which marks his position at mid- 
night, the sun being then on the meridian of places 
on the opposite part of the earth, and he is brought 
round again to D, the point of sunrise, when another 
day commences. 

Unequal Rate of Diurnal Motion. — Different 
points upon the surface of the earth revolve with 
different velocities. At the poles the speed of rota- 
tion is nothing, while at the equator it is greatest, or 
over 1,000 miles per hour. At Quito, the circle of 
latitude is much longer than the one at the mouth of 
the St. Lawrence, and the velocities vary in the 
same proportion. The former place moves at the 
rate of about 1,038 miles per hour ; the latter, 682 
miles. In our latitude (41°) the speed is about 780 
miles per hour. We do not perceive this wonderful 
velocity with which we are flying through the ether, 
because the atmosphere moves with us. * 

Were the earth suddenly to stop its rotation, the 
terrible shock would, without doubt, destroy the 

* "An ingenious inventor once suggested that we should utilize the earth's rota- 
tion, as the most simple and economical, as well as rapid mode of locomotion that could 
be conceived. This was to be accomplished by rising in a balloon to a height inacces- 
sible to aerial currents. The balloon, remaining immovable in this calm region, would 
simply await the moment when the earth, rotating underneath, should present the place 
of destination to the eyes of travelers who would then descend. A well-regulated 
watch and an exact knowledge of longitude would thus render traveling possible from 



THE EARTH. 80 

entire race of man ; while we, with houses, trees, 
rocks, and even the oceans, would be hurled, in one 
confused mass, headlong into space. On the other 
hand, were the rate of rotation to increase, the 
length of the day would be proportionately shortened, 
and the weight of all bodies decreased by the centrif- 
ugal force thus produced. If the rotary movement 
should become swift enough to reduce the day to 
eighty-four minutes, the force of gravity would be 
overcome, and, at the equator, all bodies would be 
without weight ; if the speed were still further in- 
creased, loose bodies would fly off from the eartb 
like water from a swiftly-turned grindstone, while 
we should be compelled constantly to " hold on " 
to avoid sharing the same fate.* But against such 
a catastrophe we are assured by the immutability 
of God's laws. " He is the same yesterday, to-day, 
and forever." 

Unequal Diurnal Orbits of the Stars. — In 
figure 35, let O represent our position on the earth's 
surface ; EZB, our meridian ; E I B K, our horizon ; 
P and P', the north and south poles ; Z, the zenith ; 



east to west, all voyages north or south being interdicted. This suggestion has only one 
fault ; it supposes that the atmospheric strata do not revolve with the earth. Upon 
that hypothesis, since we rotate (at London) with the velocity of 333 yards in a 
second, there would result a wind in the contrary direction ten times more violent than 
the most terrible hurricane. Is not the absence of such a state of things a convincing 
proof of the participation of the atmospheric envelope in the general movement ? "— 
Guillemin. 

* Laplace concluded in 1799 that the inequalities of the earth's rotation were too 
insignificant for measurement. But, more recently, Delaunay has shown from the moon's 
acceleration that a minute change, caused by the friction of the sea and atmosphere 
upon tbe earth's surface, has taken place, producing a variation in the length of the day. 

The acceleration of the moon in its path, is, however, only seven feet per century, 
or less than an inch per annum, and the time of the earth's rotation has increased but 
x^lo of a second in 2,400 years.— Ball. 



00 



THE SOLAR SYSTEM. 



Z', the nadir ; and G I C K the celestial equator. Now 
P B, it will be seen, is the elevation of the north pole 
above the horizon, or the latitude of the place. 



Fig. 35. 




Suppose we should see a star at A, on the meridian 
below the pole. The earth revolves in the direction 
G I C ; the star will therefore move along A L to Z, 
when it is on the meridian above the pole. It con- 
tinues its course along the dotted line around to A 
again, when it is on the meridian below the pole, 
having made a complete circuit around the pole, but 
not having descended below our horizon. 

A star rising at B would just touch the horizon ; 
one at I would move on the celestial equator, and 
would be above the horizon as long a time as it is 
below, — twelve hours in each case ; a star rising at M 
would come just above the horizon and set again at 1ST. 



the earth. 91 

Unequal Diurnal Velocities of the Stars. — 
The stars appear to us to be set in a concave shell 
which revolves daily about the earth. As differ- 
ent parts of the earth really rotate with varying ve- 
locities, so the stars appear to revolve at different 
rates of speed. Those near the pole, having a small 
orbit, revolve very slowly, while those near the 
celestial equator move at the greatest speed. 

Appearance of the Stars at Different Places 
on the Earth. — Were we placed at the north pole, 
Polaris would be directly overhead, and the stars 
would seem to pass around us in circles parallel to 
the horizon, and increasing in diameter from the 
upper to the lower ones. Were we placed at the 
equator, the pole-star would be at the horizon, and 
the stars would move in circles perpendicular to the 
horizon, and decreasing in diameter, north and south 
from those in the zenith, while we could see one 
half of the path of each star. Were we placed in the 
southern hemisphere, the circumpolar stars would 
revolve about the south pole, and the others in 
circles resembling those in our sky, only the points of 
direction would be reversed to correspond with the 
pole. Were we placed at the south pole, the appear- 
ance would be the same as at the north pole, except 
that no star is there to mark the direction of the 
earth's axis. 

Motion of the Earth in Space about the Sun. — 
The earth revolves in an elliptical path about the 
sun at a mean distance of 93,000,000 miles. 

The eccentricity of this path, which is greater 
than that of the orbit of Venus, changes about 



92 THE SOLAR SYSTEM. 

too*ooo P er century. The orbit would, therefore, 
finally become circular, were it not that, after the 
lapse of some thousands of years, the eccentricity 
will begin to increase again, and will thus vary 
through all time within definite, although yet un- 
determined limits. The circumference is nearly 
GOO, 000, 000 miles, and the earth pursues this wonder- 
ful journey at the rate of over eighteen miles per 
second. 

This revolution of the earth about the sun gives 
rise to various phenomena, of which we shall now 
proceed to speak. 

1. Change in the Appearance of the Heavens 
in Different Months. — In Fig. 36, suppose A B C D 
to be the orbit of the earth, and E F G H the 
sphere of the fixed stars, surrounding the sun in 
every direction. When our globe is at A, the stars 
about E are on the meridian at midnight. Being 
seen from the earth in the quarter opposite to 
the sun, they are favorably placed for observa- 
tion. The stars at G, on the contrary, will be in- 
visible, for the sun intervenes between them and 
the earth : they are on the meridian of the spec- 
tator about the same time as the sun, and are hidden 
in his rays. 

In three months, the earth has passed over one- 
fourth of its orbit, and has arrived at B. Stars 
about F now appear on the meridian at midnight ; 
those at E, which previously occupied their places, 
have descended toward the west ; while those about 
G are just coming into sight in the east. 

In three months more, the earth is situated at C, 



THE EARTH. 93 

and stars about G shine in the midnight sky, those 
at F having, in their turn, vanished in the west ; 

Fig. 36. 
H 




**** * * * *** 

* ' * %> 
F 

Appearance of the Heavens in Different Seasons (Hind). 

stars at E are on the meridian at noon, and conse- 
quently hidden in daylight ; and those about H are 
just making their appearance in the east. One revo- 
lution of the earth will bring the same stars again 
on the meridian at midnight. 

Thus the earth's motion round the sun as a center 
explains the varied aspect of the heavens in the 
summer and winter skies. 



94 the solar system. 

2. Yearly Path op the Sun Through the 
Heavens. — We have spoken of the diurnal motion 
of the sun. We shall now speak of its second ap- 
parent motion, its yearly path among the stars, — the 
ecliptic* If we look at Fig. 37, we can see how the 
motion of the earth in its orbit is transferred to the 
sun, and causes him to appear to travel in a fixed 
path through the heavens. When the earth is in any 
part of its orbit, the sun seems to us to be in the 
point directly opposite. For example, when the 
earth is in Libra (^)f — autumnal equinox — the sun 
is in Aries (°P) — vernal equinox; when the sun 
enters the next sign, Taurus (a), the earth has 
passed on to Scorpio (m). Thus, as the earth moves 
through her orbit, the sun seems to pass along the 
opposite side of the ecliptic, making the circuit of 
the heavens in a year, and returning, at the end of 
that time, to the same place among the stars. The 
ecliptic crosses the celestial equator at two points, 
called the equinoxes. (See page 30). 

* This yearly movement of the sun among the fixed stars is not so apparent to us as 
his daily motion, because his superior light blots out the stars. But if we notice a star 
at the western horizon just at sunset, we can tell what constellation the sun is in : then 
wait two or three nights, and we shall find that this star has set, and others have taken 
its place. Thus we can trace the sun through the year in his path among the fixed 
stars in the horizon. 

t When we say "the earth is in Libra," we mean that a spectator placed at the sun 
would see the earth in that part of the heavens which is occupied by the sign Libra, 
while a spectator on the earth would see the sun, at the same time, in that part of the 
heavens which is occupied by the sign Aries. Just so, on June 21st, the earth enters 
Capricorn, and the sun, Cancer. It is customary, however, having reference solely to 
the sun's place, to locate the vernal equinox in Aries, and the autumnal equinox in 
Libra ; the summer solstice in Cancer, and the winter solstice in Capricorn. In figure 
37, the terms "summer solstice," "autumnal equinox," etc., refer to the season 
upon the earth, and to the location of the sun in the ecliptic, but are not the names of 
those points on the earth's orbit. The zodiacal signs are inserted for convenience of 
illustration, to show where the earth would be located by a solar spectator ; the pupil 
should remember, however, that the signs belong to the ecliptic — which is the projec- 
tion of the plane of the earth's orbit upon the celestial sphere, and not to the earth's path. 



the earth. 95 

3. Apparent Movement of the Sun, North and 
South. — Having now spoken of the apparent diurnal 
and annual motions of the sun, there yet remains a 
third motion. In summer, at midday, the sun is 
high in the heavens ; in winter, he is low, near the 
southern horizon. In summer, he is a long time above 
the horizon ; in winter, a short time. In summer, he 
rises and sets north of the east and west points ; in 
winter, south of the east and west points. This 
subject is so intimately connected with the next, 
that we shall understand it best when taken in con- 
nection with that topic. 

4. Change of the Seasons. Variation in Length 
of Day and Night. — By studying Fig. 37, and 
imagining the various positions of the earth in 
its orbit, let us try to understand the following 
points : 

I. Obliquity of the ecliptic. — The axis of the earth 
is inclined 23^° from a perpendicular to its orbit. 
This angle is called the obliquity of the ecliptic. 

II. Parallelism of the axis. — In all parts of the 
orbit, the axis of the earth is parallel to itself, and 
points almost exactly toward the North Star (p. 217). 

Nature reveals to us nothing more permanent than 
the axis of rotation in anything that is rapidly 
turned. It is its rotation that keeps a boy's hoop 
from falling. For the same reason, a quoit retains 
its direction when whirled, and stays in the same 
plane at whatever angle it may be thrown. A man 
slating a roof wishes to throw a slate to the ground ; 
he whirls it perpendicularly, and it will strike on the 
edge without breaking. So long as a top spins there 



Fig. 87. 




The Orbit of the Earth as seen by an Observer at the Sun. (See Note, p. 9k.) 



THE EARTH. 97 

is no danger of its falling, since its tendency to keep 
its axis of rotation parallel is greater than the attrac- 
tion of the earth. This wonderful law would lead 
us to think that the axis of the earth always points 
in the same direction, even if we did not know it 
from direct observation. 

III. The rays of the sun strike the various portions 
of the earth, when in any position, at different 
angles. — When the earth is in Libra, and also when 
in Aries, the sun's rays strike vertically at the equa- 
tor, and more and more obliquely in the northern 
and southern hemispheres, as the distance from the 
equator increases, until at the poles they strike 
almost horizontally. 

. This variation in the direction of the rays pro- 
duces a corresponding variation in the intensity of 
the sun's heat and light at different places, and 
accounts for the difference between the torrid and 
polar regions. 

IY. As the earth changes its position the angle at 
which the rays strike any portion is varied. — Take 
the earth when it is in Capricornus (V?) and the sun 
in Cancer (s). He is now overhead, 23J° north of 
the equator. His rays strike less obliquely in the 
northern hemisphere than when the earth was in 
Libra. Let six months elapse : the earth is now in 
Cancer and the sun in Capricornus ; and* he is over- 
head, 23^° south of the equator. His rays strike less 
obliquely in the southern hemisphere than before, 
but in the northern hemisphere more obliquely. 
These six months have changed the direction of the 
sun's rays on every part of the earth's surface. This 



98 THE SOLAR SYSTEM. 

accounts for the difference in temperature between 
summer and winter.* 

V. Equinoxes. — At the equinoxes, one half of each 
hemisphere is illuminated : hence the name Equinox 
(cequus, equal ; and nox, night). At these points of 
the orbit, the days and nights are equal over the 
entire earth, f each being twelve hours in length. 

VI. Northern and southern hemispheres unequally 
illuminated. — While one-half of the earth is con- 
stantly illuminated, the proportion of the northern 
or the southern hemisphere that is in daylight or 
darkness varies at all times, except at the equinoxes. 
When more than half of a hemisphere is in the light, 
its days are longer than the nights, and vice versa. 

VII. The seasons and the comparative length of 
the days and nights in the South Temperate Zone, at 
any time, are the reverse of those in the North Tem- 
perate Zone, except at the Equinoxes, when the days 
and nights are of equal length. 

VIII. The Summer Solstice. — At the time of the 
summer solstice, which occurs about the 21st of 
June, the sun is overhead 23J° north of the equator, 
and if his vertical rays could leave a golden line on 
the surface of the earth as it rotates, they would 
mark the Tropic of Cancer. The sun is at its fur- 
thest northern declination ; he ascends the highest 
he is ever Seen above our horizon, and rises and sets 
north of the east and west points. He seems now 
to stand still in his northern and southern course. 



* The long nights and short days of winter, and the short nights and long days of 
summer, are also important factors in producing this difference of temperature, 
t Except a small space at each pole, 



THE EARTH. 99 

and hence the name Solstice (sol, the sun ; sto, I 
stand). The days in the north temperate zone are 
longer than the nights. It is our summer, and the 
21st of June is the longest day of the year. 

In the south temperate zone it is winter, and the 
shortest day of the year. The circle that separates 
day from night extends 23^° beyond the north pole, 
and if the sun's rays could in like manner leave a 
golden line on that day, they would trace on the 
earth the Arctic Circle. It is the noon of the long 
six-months polar day. The reverse is true at the 
Antarctic Circle, and it is there the midnight of the 
long six-months polar night (p. 117). 

IX. The Autumnal Equinox.— The earth crosses 
the aphelion point about the 1st of July. It is then 
at its furthest distance from the sun, which each 
day rises and sets a trifle further toward the south, 
passing through a lower circuit in the heavens. At 
the time of the autumnal equinox,* the 22nd of Sep- 
tember, he is on the equinoctial, and if his vertical 
rays could leave a line of golden light, they would 
mark on the earth the circle of the equator. It is 
autumn in the north temperate zone and spring in 
the south temperate zone. The days and nights 
are equal over the whole earth, the sun rising at 
6 a.m. and setting at 6 p.m., exactly in the east and 
the west, where the equinoctial intersects the horizon. 

X. The Winter Solstice. — The sun after passing 
the equinoctial — ■" crossing the line" — sinks lower 
toward the southern horizon each day. At the 

* The precise time of the equinoxes and solstices varies each year, hut within a small 
limit. 



100 THE SOLAR SYSTEM. 

time of the winter solstice, about the 21st of De- 
cember, the sun is directly overhead 23 J° south of 
the equator, and if his vertical rays could leave a 
line of golden light, they would mark on the earth's 
surface the Tropic of Capricorn. He is at his furthest 
; southern declination, and rises and sets south of the 
east and west points. It is our winter, and the 21st 
vof December is the shortest day of the year. 

In the south temperate zone it is summer, and 
the longest day of the year. The circle that separates 
day from night extends 23^° beyond the south pole, 
and if the sun's rays in like manner could leave a 
line of golden light, they would mark the Antarctic 
Circle. It is there the noon of the long six-months 
polar day. At the Arctic Circle the reverse is true ; 
the rays fall 23J° short of the north pole, and it is 
there the midnight of the long six-months polar 
night. Here again the sun appears to us to stand 
still a day or two before retracing his course, and it 
is therefore called the Winter Solstice. 

XI. The Vernal Equinox. — The earth reaches its 
perihelion about the 31st of December. It is then 
nearest the sun, which rises and sets each day fur- 
ther and further north, and climbs up higher in the 
heavens at midday. Our days gradually increase in 
length, and our nights shorten in the same propor- 
tion. About the 21st of March the sun reaches the 
equinoctial, at the vernal equinox. He is overhead 
at the equator, and the days and nights are again 
equal. It is our spring, but in the south temperate 
zone it is autumn. 

XII. Yearly path finished. — The earth moves on in 



THE EARTH. 101 

its orbit through the spring and the summer months. 
The sun continues his northerly course, ascending 
each day higher in the heavens, and his rays becom- 
ing less and less oblique. About the 21st of June, he 
again reaches his furthest northern declination, and 
is at the summer solstice. 

We have thus traced the yearly path, and noticed 
the course of the changing seasons, with the length 
of the days and nights. The same series has been 
repeated through the ages of the past, and will be 
through the future till time shall be no more. 

XIII. Distance of the earth from the sun varies. — 
We notice, from what we have just seen, that we are 
nearer the sun in winter than in summer by 3,000,000 
miles. The obliqueness with which the rays strike 
the north temperate zone at that time prevents our 
receiving any special benefit from this favorable 
position of the earth. 

XIV. Southern summer. — The inhabitants of the 
south temperate zone have their summer while the 
earth is in perihelion, and the sun's rays are about 
•jV warmer than when in aphelion, our summer-time. 
This will perhaps partly account for the extreme 
heat of their season. * The southern winters, for a 
similar reason, are colder ; and this makes the aver- 
age yearly temperature about the same as ours. 

XV. Extremes of heat and cold not at the solstices. 
— We do not have our greatest heat at the time of the 
summer solstice, nor our greatest cold at the winter 

* Captain Sturt, in speaking of the extreme heat of Australia, says that matches acci- 
dentally dropped on the ground were ignited. A recent official report states that, in 
South Australia, January, 1882, the heat, in the sun, was 180°— only 32° below the 
boiling-point. 



102 THE SOLAR SYSTEM. 

solstice. After the 21st of June, the earth, already 
warmed by the genial spring days, continues to 
receive more heat from the sun by day than it radi- 
ates by night : thus its temperature still increases. 
On the other hand, after the 21st of December, the 
earth continues to become colder, because it loses 
more heat during the night than it receives during 
the day. 

XVI. Summer longer than winter. — As the sun is 
not in the center of the earth's orbit, but at one of 
its foci, the earth, from the time of the vernal to that 
of the autumnal equinox, passes through more than 
one-half of its orbit. The summer is, therefore, longer 
than the winter. The difference is enhanced by the 
variation in the earth's velocity at aphelion and at 
perihelion. 

XVII. Varying velocity of earth. — From the time 
of the vernal equinox until the earth passes its 
aphelion, the solar attraction tends to check its 
speed ; thence until the time of the autumnal equinox, 
the attraction is partly in the direction of its motion, 
and so increases its velocity. The same principle 
applies when going to and from perihelion. 

XVIIL Curious appearance of the sun at the north pole. — " To a person 
standing at the north pole, the sun appears to sweep horizontally around 
the sky every twenty -four hours, without any perceptible variation in its 
distance from the horizon. It is, however, slowly rising, until, on the 21st 
of June, it is twenty-three degrees and twenty-eight minutes > above the 
horizon, a little more than one-fourth of the distance to the zenith. This 
is the highest point it ever reaches. From this altitude, it slowly descends, 
its track being represented by a spiral or screw with a very fine thread ; 
and in the course of three months it worms its way down to the horizon, 
which it reaches on the 22nd of September. On this day it slowly sweeps 



THE EARTH. 103 

around the sky, with its face half hidden below the icy sea. It still con- 
tinues to descend, and after it has entirely disappeared it is still so near the 
horizon that it carries a bright twilight around the heavens in its daily 
circuit. As the sun sinks lower and lower, this twilight grows gradually 
fainter, till it fades away. December 21st, the sun is 23£° below the 
horizon, and this is the midnight of the dark polar winter. From this 
date, the sun begins to ascend, and after a time it is heralded by a faint 
dawn, which circles slowly around the horizon, completing its circuit every 
twenty-four hours. This dawn grows gradually brighter, and on the 22nd 
of March the peaks of ice are gilded with the first level rays of the six- 
months day. The bringer of this long day continues to wind his spiral 
way upward, till he reaches his highest place on the 21st of June, and his 
annual course is completed. " 

XIX. Results, if the axis of the earth were perpen- 
dicular to the ecliptic. — The sun would then always 
appear to move through the equinoctial. He would 
rise and set every day at the same points on the 
horizon, and pass through the same circle in the 
heavens, while the days and nights would be equal 
the year round. There would be near the equator a 
fierce torrid heat, while north and south the climate 
would change into temperate spring, and, lastly, 
into the rigors of a perpetual winter. 

XX. Results, if the equator of the earth were per- 
pendicular to the ecliptic. — Were this the case, to 
a spectator at the equator, as the sun leaves the 
vernal equinox, he would each day pass through 
a smaller circle, until at the summer solstice he 
would reach the north pole, when he would halt for 
a time, and then slowly return in an inverse man- 
ner. 

In our own latitude, the sun would make his 
diurnal rotations as described, his rays shining 



104 THE SOLAR SYSTEM. 

past the north pole further and further, until we 
were included in the region of perpetual day, when 
he would seem to wind in a spiral course up to the 
north pole, and then return in a descending curve to 
the equator. 

Precession of the Equinoxes. — We have spoken 
of the equinoxes as if they were stationary. Over 
two thousand years ago, Hipparchus (see page 8) 
found that they are slowly falling back along the 
ecliptic. Modern astronomers fix the rate at about 
50" of space annually. If we mark either point in 
the ecliptic where the days and nights are equal 
over the earth — at which time the plane of the 
earth's equator passes exactly through the center of 
the sun — we shall find the sun comes back to that po- 
sition the next year, 50" (20 m. 20 s. of time) earlier. 
This remarkable effect is called the Precession of the 
Equinoxes, because the position of the equinoxes in 
any year precedes that which they occupied the year 
before. Since the circle of the ecliptic is divided 
into 360°, it follows that the time occupied by the 
equinoctial points in making a complete revolution 
at the rate of 50".2 per year is 25,817 years. 

Results of the Precession of the Equinoxes. — 
In Fig. 37, we see that the plane of the earth's 
equator is inclined to that of the ecliptic. In order 
that the plane of the terrestrial equator should pass 
through the sun's center 50" earlier, it is necessary 
that the plane itself should slightly change its place. 
The axis of the earth is always perpendicular to 
this plane, hence it follows that the axis is not rigor- 
ously parallel to itself. It varies in direction, so that 



THE EARTH. 



105 



the north pole describes a small circle in the starry 
vault twice 23^° in diameter. 

To illustrate this, let us suppose that, after a series 
of years, the position of the earth's equator has 
changed from e/ ft to gKl (Fig. 38). The inclina- 
tion of the axis of the earth, CP, to CQ, the pole of 
the ecliptic, remains unchanged ; but as it must turn 




Change of Earth's Equator and Axis.* 

with the equator, its position is moved from CP to 
CP', and the pole of the earth slowly traces the por- 
tion of a circle, PP'. The direction of this motion is 
the same as that of the hands of a watch, or the re- 
verse of the revolution of the earth. The position of 
the north pole in the heavens is gradually but almost 
insensibly changing. It is now distant from the north 
polar star about 1|°. It will continue to approach it 

* See in the Appendix a description of a simple apparatus for illustrating this 
subject. 



106 THE SOLAR SYSTEM. 

until they are not more than half a degree apart. 
In 12,000 years, Lyra will be our polar star : 4,500 
years ago the polar star was the bright star Alpha 
in the constellation Draco. (See p. 217). 

As the right ascension of the stars is reckoned 
eastward from the vernal equinox along the equi- 
noctial, the precession of the equinoxes increases the 
R. A. of the stars 50" per year. On this account, star 
maps should be accompanied by the date of their 
calculation, that they may be corrected to corre- 
spond with this annual variation. 

The constellations of the zodiac (see p. 31) are 
fixed in the heavens, while the signs are simply 
abstract divisions which move with the equinox. 
When named, the sun was in both the sign and 
the constellation Aries, at the time of the vernal 
equinox ; but since then the equinoxes have retro- 
graded nearly a whole sign, so that now, while the 
vernal equinox is in the sign Aries, this sign cor- 
responds to the constellation Pisces, which is there- 
fore the first constellation in the zodiac (Fig. 8G). 

Causes of the Precession of the Equinoxes.— 
Before commencing the explanation of this phenom- 
enon, it is necessary to impress upon the mind a few 
facts. (1.) The earth is not a perfect sphere, but is 
swollen at the equator. It is like a sphere covered 
with padding, increasing in thickness from the poles 
to the equator ; this gives it a turnip-like shape. 
(2.) The attraction of the sun is greater the nearer 
a body is to it. (3.) The attraction is not for the 
earth as a mass, but for each particle separately. 

In the figure, the position of the earth at the time 



THE EARTH. 



107 



of the winter solstice is represented. P is the north 
pole ; a b, the plane of the ecliptic ; C, the center of 
the earth ; CQ,a line perpendicular to the ecliptic ; 
the angle Q C P, the obliquity of the ecliptic. In this 
position, the equatorial padding of which we have 
spoken — the ring of matter about the equator — is not 
turned exactly toward the sun, but is elevated above 
it. Now the attraction of the sun pulls the part D 
more strongly than the center ; the tendency of this 

Fig. 89. 




Influence of the Sun on a Mountain near the Equator. 




is to bring D down to a, and to lift I toward b. The 
attraction for C is greater than for I, so it tends to 
draw C away from I, and, as at the same time D 
tends toward a, to pull I up toward b. The effect 
of this, one would think, would be to change the 
inclination of the axis C P toward C Q, and make it 
more nearly perpendicular to the ecliptic. This 
would be the result if the earth were not rotating 
upon its axis. 

Let us consider the case of a mountain near the 
equator. This, if the sun did not act upon it, would 
pass through the curve H D E in the course of a 
semi-rotation of the earth. But, it is nearer the sun 



108 THE SOLAR SYSTEM. 

than is the center C ; the attraction therefore tends 
to pull the mountain downward and tilt the earth 
over, as we have just described ; so the mountain 
will pass through the curve H/g, and, instead of 
crossing the ecliptic at E, will cross at g, a little 
sooner than it otherwise would. The same influence, 
though in a less degree, obtains on the opposite side 
of the earth. The mountain passes around the earth 
in a curve nearer to b, and crosses the ecliptic a 
little earlier. 

The same reasoning will apply to each mountain 
and to all the protuberant mass near the equatorial 
regions. The final effect is slightly to turn the 
earth's equator so that it intersects the ecliptic 
sooner than it would were it not for this attraction. 
At the summer solstice, the same tilting motion is 
produced. At the equinoxes, the plane of the earth's 
equator passes through the center of the sun, and 
therefore there is no tendency to change of position. 
As the axis C P must move with the equator, it 
slowly revolves, keeping its inclination unchanged, 
around C Q, the pole of the ecliptic, describing, in 
about 26,000 years, a small circle twice 23^° in diam- 
eter. 

Precession illustrated in the spinning of a top. 
— This motion of the earth's axis is singularly illus- 
trated in the spinning of a top, and the more so 
because the forces are of an opposite character to 
those which act on the earth, and thus produce an 
opposite effect. We have seen that, if the earth had 
no rotation, the sun's attraction on the "padding" 
at the equator would bring C P nearer to C Q, but 



THE EARTH. 



109 




that, in consequence of this rotation, the effect 
really produced is that C P, the earth's axis, slowly 
revolves around C Q, the pole of the heavens, in a 
direction opposite to that of rotation. 

In Fig. 40, let C P be the axis of a spinning top, 
and C Q the vertical line. 
The direct tendency of 
the earth's attraction is 
to bring C P further from 
C Q (or to make the top 
fall), and if the top were 
not spinning this would 
be the result ; but, in con- 
sequence of the rotary 
motion, the inclination 
does not sensibly alter 
(until the spinning is re- 
tarded by friction), and so C P slowly revolves 
around C Q in the same direction as that of rotation. 

Nutation (nutatio, a nodding). — We have noticed 
the sun as producing precession ; the moon has, 
however, treble his influence ; for although her mass 
is not -g-5,ooV¥oo P ar ^ that of the sun, yet she is 400 
times nearer and her effect correspondingly greater. * 
The moon's orbit does not lie parallel to the ecliptic, 
but is inclined to it. Now the sun attracts the moon, 
and disturbs its path, as he would that of the moun- 
tain we have supposed, and the effect is the same. 
The intersections of the moon's orbit with the ecliptic 
travel backward, completing a revolution in about 
18 years. 

* See the Differential effect of the Sun and the Moon, under the head of the Tides. 



Spinning of a Top. 



no 



THE SOLAR SYSTEM. 



Fig. 14. 




Path of the North Pole in the 
Heavens. 



During half of this time, the moon's orbit is in- 
clined to the ecliptic in the same way as the earth's 
equator ; during the other half, it is inclined in the 
opposite way. In the former state, the moon's at- 
tractive tendency to tilt the earth is very small, and 
the precession is slow ; in the latter, the tendency is 
great, and precession goes on 
rapidly. The consequence of this 
is, that the pole of the earth is 
irregularly shifted, so that it 
travels in a slightly curved line, 
giving it a kind of "wabbling" 
or "nodding" motion, as shown 
— though greatly exaggerated — 
in Fig. 41. The obliquity of the 
ecliptic, which we consider 23£° 
(23° 27' 15", Jan. 1, 1884. See p. 29), is the mean of 
the irregularly curved line and is represented by 
the dotted circle. 

Periodical change in the obliquity of the 
ecliptic. — Although it is sufficiently near for all 
general purposes to consider the obliquity of the 
ecliptic invariable, yet this is not strictly the case. 
It is subject to a small but appreciable variation of 
about 4G" per century. This is caused by a slow 
change of the position of the earth's orbit, due to the 
attraction of the planets. The effect of this move- 
ment is gradually to diminish the inclination of the 
earth's equator to the ecliptic (the obliquity of the 
ecliptic). This will continue for a time, when the 
angle will as gradually increase ; the extreme limit 
of change being only 1° 21', The orbit of the earth 



THE EARTH. Ill 

thus vibrates backward and forward, each oscillation 
requiring a period of 10,000 years. 

The change is so intimately blended, in its effect 
upon the obliquity of the ecliptic, with that caused 
by precession and nutation, that they are separable 
only in theory ; in fact, they all combine to produce 
the waving motion we have already described. As 
a consequence of this variation in the obliquity of 
the ecliptic, the sun does not now come so far north 
nor decline so far south as formerly ; while the 
position of all the terrestrial circles — the Tropics of 
Cancer, Capricorn, etc. — is constantly but slowly 
changing. As the result of this variation in the 
position of the orbit, some stars which were once 
just south of the ecliptic are now north of it, and 
others that were just north are now a little further 
north ; thus the latitude of these stars is gradually 
changing. 

Change in the major axis (line of apsides) op 
the earth's orbit. — Besides all the changes in the 
position of the earth in its orbit due to precession, 
etc., the line connecting the aphelion and peri- 
helion points of the orbit itself is slowly revolving. 
The consequence of this is a variation in the length 
of the seasons at different periods of time. 

In the year 3958 B.C., the earth was in perihelion at the time of the au- 
tumnal equinox, so that the summer and autumn seasons were of equal 
length, but shorter than the winter and spring seasons, which were also equal. * 

* There is much discrepancy in the views held concerning the Great Year of the 
astronomers, as it is often called. (See Steele's Geology, pp. 272-3, note.) The state- 
ment made in the text is that held by Lockyer, Hind, and others. The dates are those 
given by Chambers in his Descriptive Astronomy (3rd Edition), where the subject is fully 
described. 



112 THE SOLAR SYSTEM. 

In the year 1267 A.D., the earth was in perihelion at the time of 
the winter solstice, December 21, instead of January 1st, as now ; the 
spring quarter was therefore equal to the summer one, and the autumn 
quarter to the winter one, the former being the longer. In the year 6493 
a. d. , the earth will be in perihelion at the time of the vernal equinox ; 
summer will then be equal to autumn and winter to spring, the former 
seasons being the longer. In the year 11719 a.d., the earth will be in 
perihelion at the time of the summer solstice : finally, in 16945 a.d., the 
cycle will be completed, and the autumnal equinox will again coincide with 
the earth's perihelion. 

Permanence in the Midst of Change. — We thus 
see that the ecliptic is constantly modifying its ellip- 
tical shape ; that the orbit of the earth slowly oscil- 
lates upward and downward ; that the north pole 
steadily turns its long index-finger over a dial that 
marks 26,000 years ; that the earth, accurately 
poised in space, gently nods and bows to the attrac- 
tion of sun, moon, and planets. * Thus changes are 
taking place that would ultimately entirely reverse 
the order of nature. But each of these variations 
has its bounds, beyond which it cannot pass. The 
promise made to man is that, "while the earth re- 
maineth, seed-time and harvest, and cold and heat, 
and summer and winter, and day and night shall 
not cease." The modern discoveries of astronomy 
prove conclusively that the seasons are to be perma- 
nent ; that the Creator, amid all these transitions, 
has ordained the means of carrying out His promise 
through all time. 

Refraction. — The atmosphere extends above the 

* These oscillations extend throughout the whole planetary system, the periods 
varying from 50,000 to 2,000,000 years. "Great clocks of eternity, which beat ages as 
ours beat seconds." — Newcomb's Astronomy, page 95. 



THE EARTH. 



113 



earth about 500 miles (Physics, p. 116). Near the 
surface it is dense, while in the upper regions it is 



Fig. IS. 




Refraction. 

exceedingly rare. The rays of light from the heav- 
enly bodies passing through these different layers 
are turned downward toward a perpendicular more 
and more as the density increases. According to a 
well-known law of optics (Physics, p. 150), if the ray 
of light from a star were bent in fifty directions 
before entering the eye, the star would nevertheless 
appear to be in the line of the one nearest the eye. 
The effect of this is, that the apparent place of a 
heavenly body is higher than the true place. The 
sun at S (Fig. 42), were it not for the atmosphere, 
would send a direct ray to L. Instead, the ray at 
A is refracted downward, and would then enter the 
eye at N ; passing, however, through a layer of a dif- 



114 



THE SOLAR SYSTEM. 



ferent density at B, it is again bent, and meets the 
eye of the observer at C. He sees the sun, not in the 
direction of the curved line C B A S, but in that of 
the straight line C B SI 

The amount of refraction varies with the tempera- 
ture, moisture, and other conditions of the atmos- 
phere. It is zero for a body in the zenith, and in- 
creases gradually toward the horizon (as the thick- 
ness of the intervening atmosphere increases), where 
it is sometimes as much as 35'. 

Fig. US. 




Change of Place and Appearance of the Sun 
and the Moon. — The sun may be really below the 
horizon, and yet seem to be above it. For example, 
on April 20, 1837, the moon was eclipsed before the 
sun had set. The mean diameter of both the sun 
and the moon being about half a degree, it follows 
that when we see the lower edge of either of these 
luminaries apparently just touching the horizon, in 



THE EARTH. 



115 



reality the whole disk is below it, and would be 
hidden were it not for the refraction. The day is 
consequently materially lengthened. 




Deformation of tlie. Sun near the Horizon. 

The sun and the moon often appear flattened when 
near the horizon. The rays from the lower edge 
pass through a denser layer of the atmosphere, and 
are therefore refracted more than those from the 
upper edge : the effect of this is to make the vertical 
diameter appear less than the horizontal, and so to 
distort the figure of the disk into an oval shape. 

The dim and hazy appearance of the heavenly 
bodies when near the horizon is caused not only by 
the rays of light having to pass a greater distance 
through the atmosphere, but also by their traversing 
the denser part. The intensity of the solar light is 
so greatly diminished by going through the lower 
strata, that we are then enabled to look upon the 
sun without being dazzled by his brilliant beams. 



116 THE SOLAR SYSTEM. 

Twilight.— The glow of light after sunset and 
before sunrise, which we term twilight, is caused by 
the refraction and the reflection of the sun's rays by 
the atmosphere. For a time after the sun has really 
set, the refracted rays continue to reach the earth ; 
but when these have ceased, he still illuminates the 
clouds and upper strata of the air, just as he may 
be seen shining on the summits of lofty moun- 
tains long after he has disappeared from the view of 
the inhabitants of the plains below. The air and 
clouds thus illuminated reflect back a part of the 
light to the earth. As the sun sinks lower, less light 
reaches us, until reflection ceases and night ensues. 
The same thing occurs before sunrise, only in 
reverse order. 

Twilight is usually reckoned to last until the de- 
pression of the sun below the horizon amounts 
to 18° ; this, however, varies with the latitude,* 
seasons, and condition of the atmosphere. In the 
latitude of New York, twilight lasts from 1\ to 2 
hours, the shortest twilight being in winter, and the 
longest in summer. Strictly speaking, in the latitude 
of Greenwich there is no true night for a month 
before and after the summer solstice, but constant 
twilight from sunset to sunrise. The sun is then 
near the Tropic of Cancer, and does not descend so 
much as 18° below the horizon during the entire 
night. At the equator the length of the evening 
twilight is about 1| hours, and remains almost con- 



* When the sun's path is very oblique to the horizon, a longer time is required for 
the sun to descend or ascend the requisite vertical distance of 18° from the horizon ; 
and a shorter time, when his path is more nearly perpendicular. 



I 



THE EARTH. 117 

stant the entire year. The twilight is longest toward 
the poles, where the night of six months is shortened 
by an evening twilight of about fifty days and a 
morning one of equal length. 

Diffused Light. — The diffused light of day is pro- 
duced in the same manner as that of twilight. The 
atmosphere reflects and scatters the sunlight in 
every direction. Were it not for this, no object 
would be visible to us out of direct sunshine ; every 
shadow of a passing cloud would be pitchy dark- 
ness ; the stars would be visible all day ; no window 
would admit light except as the sun shone directly 
through it, and a man would require a lantern to go 
around his house at noon. 

The blue light reflected to our eyes from the at- 
mosphere above us, or, more correctly, from the 
vapor in the air, produces the optical illusion we 
call the sky. Were it not for this, every time we 
cast our eyes upward we should feel like one gazing 
over a dizzy precipice ; while now the crystal dome 
of blue smiles down upon us so lovingly and beauti- 
fully that we call it heaven. 

Aberration of Light. — We have seen that the 
places of the heavenly bodies are apparently changed 
by refraction. Besides this, there is another change 
due to the motion of light combined with the motion 
of the earth in its orbit. For example : the mean 
distance of the earth from the sun is about 93,000,000 
miles, and since light travels a little over 186,000 
miles per second, it follows that the time occupied 
by a ray of light in reaching us from the sun is 
about 8J- min. (8 min. 18 sec.) ; so that, in fact, 



118 



THE SOLAR SYSTEM. 



(1), we do not see the sun as it is, but as it was 8J- 
minutes ago. And since, during this time, the earth 
has moved in its orbit about 20 i" (2), we do not see 
that luminary in the exact place it occupies at the 
time of observation. 

Illustration. — Suppose a ball let fall from a point 
P, above the horizontal line A B, and a tube, of 
which A is the lower extremity, placed to receive it. 




Aberration of Light. 

If the tube were fixed, the ball would strike it on the 
lower side ; but if the tube were carried forward in 
the direction A B, with a velocity properly adjusted 
at every instant to that of the ball, while preserving 
its inclination to the horizon, so that when the ball, 
in its natural descent, reached B, the tube would 
have been carried into the position B Q, it is evident 
that the ball throughout its whole descent would be 
found in the tube ; and a spectator referring to the 



THE EARTH. 119 

tube the motion of the ball, and carried along with 
the former, unconscious of its motion, would think 
that the ball had been moving in an inclined direc- 
tion, and had come from Q. 

A very common illustration may be seen almost 
any rainy day. Choose a time when the air is quiet, 
and the drops large. Then, if you stand still, you 
will see that the drops fall vertically ; but if you 
walk forward, you will see the drops fall as if they 
were meeting you. If, however, you walk backward, 
you will observe that the drops fall as if they were 
coming from behind you. We thus see that the 
drops have an apparent as well as a real motion. 

The general effect of aberration is to cause 
each star apparently to describe in the course of a 
year a minute ellipse, the central point of which is 
the place the star would actually occupy were our 
globe at rest. 

Parallax is the difference in the direction of an ob- 
ject as seen from two different places. For a simple 
illustration, hold your finger before you in front of 
the window. Upon looking at it with the left eye 
only, you will locate your finger at some point on 
the window ; on looking with the right eye only, you 
will locate it at an entirely different point. Use your 
eyes alternately and quickly, and you will be aston- 
ished to see how your finger will seem to change its 
place. Now, the difference in the direction of your 
finger as seen from the two eyes is its parallax. 

In astronomical calculations, the position of a 
body as seen from the earth's surface is called its 
apparent place, while that in which it would be seen 



120 



THE SOLAR SYSTEM. 



from the center of the earth is called its true 
place. Thus, in Fig. 46, a star is seen by the ob- 
server at O in the direction OP; if it could be 
viewed from the center R, its direction would be 
in the line R Q. It is therefore seen from O at a 
point in the heavens below its position in reference 
to R. From looking at the cut, we can see (1), that 




the parallax of a star near the horizon is greatest, 
while it decreases gradually until it disappears alto- 
gether at the zenith, since an observer at O, as well 
as one at R, would see the star Z directly overhead \ 
and (2), that the nearer a body is to the earth the 
greater its parallax becomes. 

It has been agreed by astronomers, for the sake of 
uniformity, to correct all observations so as to refer 



THE EARTH. 121 

them to their true places as seen from the center of 
the earth. Tables of parallax are constructed for 
this purpose. The question of parallax is also of 
great importance, because as soon as the parallax of 
a body is accurately known, its distance, diameter, 
etc., can be determined. (See Celestial Measure- 
ments.) 

Horizontal Parallax is the parallax of a body 
when at the horizon. It is, in fact, the earth's semi- 
diameter as seen from the body. In Fig. 46, the 
parallax of the star S is the angle S R, which is 
measured by the line R — the semi-diameter of the 
earth. The sun's horizontal parallax is the angle 
subtended (measured) by the earth's semi-diameter 
as seen from that luminary. As the moon is nearest 
the earth, its horizontal parallax is greater than that 
of any other heavenly body. 

Annual Parallax. — The fixed stars are so dis- 
tant from the earth that they exhibit no change of 
place when seen from different parts of the earth. 
The lines O S and R S are so long that they are 
apparently parallel. Astronomers, therefore, instead 
of taking the earth's semi-diameter, or 4,000 miles, 
as the measuring tape, observe the position of the 
fixed- stars at opposite points in the earth's orbit. 
This gives a change in place of 186,000,000 miles. 
The variation of position which the stars undergo at 
these remote points is called their annual parallax. 

6 



122 



THE SOLAR SYSTEM. 



THE MOON. 

First Quarter, ®. Full Moon, ©. Last Quarter, •. 



Motion in Space. — The orbit of the moon, consid- 
ering the earth as fixed, is an ellipse of which our 
planet occupies one of the foci. Her distance from 



Fig. U7. 




Path of the Moon. 



the earth, therefore, varies incessantly. At perigee 
(peri, near ; ge, the earth), she is 26,000 miles nearer 
than in apogee (apo, from j ge, the earth) : the mean 
distance is about 239,000 miles. To reach the moon, 



THE MOON. 123 

would require a chain of thirty globes equal in size 
to the earth. An ordinary express-train would take 
about a year to accomplish the journey. 

The moon completes her revolution (sidereal) 
around the earth in about 27-| days ; but, as the 
earth is constantly passing on in its orbit around 
the sun, it requires over two days longer before the 
moon comes into the same position with respect to 
the sun and the earth, thus completing a synodic 
revolution, or lunar month (291- days). 

The real path of the moon is the result of her 
own motion and the onward movement of the earth. 
The two combined produce a wave-like curve that 
crosses the earth's path twice each month ; this, 
owing to its small diameter compared with that of 
the earth's orbit, is always concave toward the sun. 
As the moon constantly keeps the same side turned 
toward us, it follows that she must rotate on her 
axis once each month. 

Dimensions. — The moon's diameter is about 2,160 
miles. To equal the earth, would require fifty globes 
the size of the moon. The apparent size varies with 
the distance ; the mean is, however, about one-half 
a degree, nearly the same as that of the sun. The 
moon always appears larger than she really is, on 
account of her brightness. This is the effect of what 
is termed in optics Irradiation* For the same 
reason it is often noticed that the crescent moon 
seems to be a part of a larger cir'cle than the rest of 



* To illustrate this principle, cut two circular pieces of the same size, one of black 
and the other of white paper. The white circle, when held in a bright light, will appear 
much larger than the black one. 



. . V 



124 THE SOLAR SYSTEM. 

the moon. The moon appears larger on the horizon 
than when high in the sky. This, however, is a 
mere illusion.* By an examination of the cut, it is 
easily seen that the moon is 4,000 miles nearer when 
on the zenith than when at the horizon. 

Besides these general variations in size, the moon 
varies in apparent size to different observers. Much 

Fig. IS. 




The Distance of the Moon at the Horizon and at the Zenith. 

amusement may be had in a large party or class by 
a comparison of her apparent magnitude. The esti- 
mates will differ from a small saucer to a wash-tub. 

Librations (librans, swinging). — Though the moon 
presents the same hemisphere to us, there are three 
causes which enable us to see, in all, about T fwo of 
her entire surface. 

1. The axis of the moon is inclined a little to her 
orbit, as also her orbit is inclined to the earth's orbit ; 
so, when her north pole leans alternately toward and 

* At the horizon we compare her with various terrestrial objects which lie between her 
and us, while aloft we have no association to guide us in judging of her distance, and we 
are led to underrate her size. If we look at her when near the horizon, through a, roll of 
paper, or the hands held tube-wise, this illusion will vanish. ._.'..... 



the moon. 125 

from the earth, we see sometimes past her north and 
sometimes past her south pole. This is called libra- 
tion in latitude. 

2. The moon's rotation on her axis is always per- 
formed in the same time, while her movement along 
her orbit is variable ; hence we occasionally see a 
little further around each limb (outer edge) than at 
other times. This is called libration in longitude. 

3. The size of the earth is so much greater than 
that of the moon, that an observer, by the rotation 
of the earth, or by going north or south, can see fur- 
ther around the limbs. 

Light and Heat. — If the whole sky were covered 
with full moons, they would scarcely make daylight, 
since the brilliancy of the moon does not exceed 
6-oo?oiro that of the sun. That portion of the moon's 
surface which is directly exposed to the sun has 
been thought to be highly heated, possibly to the 
degree of boiling water,* but this is now considered 
very improbable. 

Whether or not the moon radiates any heat to the 
earth has long been a mooted question. The best 
authorities, at present, estimate the average heat of 
the moonbeam at about -^-^ :ww that we receive from 
the sun, or sufficient to raise the temperature of a 
sensitive black-bulb thermometer -^oV <r °f a degree. 



* Prof. Langley is now engaged in an exhaustive series of experiments upon this sub- 
ject, using his famous "bolometer" — an instrument capable of detecting a difference of 
.00001° C. The result of his observations upon Mount Whitney (1881) showed that " mer- 
cury would remain a solid under the vertical rays of a tropical sun were radiation into 
space wholly unchecked, and that the temperature of a planet may, and not improbably 
does, depend far less upon its neighborhood to, or remoteness from, the sun, than upon 
the constitution of its atmosphere." As the moon has no air-blanket, it is therefore very 
doubtful whether its surface ever reaches a temperature of —100° F. _ 



126 



THE SOLAR SYSTEM. 



It would be absurd to suppose that this slight amount 
of heat can have any appreciable effect upon the 
weather. 

Center of Gravity.— It is thought that possibly the 
center of gravity of the moon is not exactly at her 
center of magnitude, but about thirty-three miles 
beyond, the lighter half being toward us. If that be 
so, this side is equivalent to a mountain of that 
enormous height ; and if water and air exist upon 
the moon, they cannot remain on this hemisphere, 
but must be confined to the side which is forever 
hidden from our view. 

Atmosphere of the Moon.— The existence of an 

atmosphere upon our satellite is at present an open 

question. If there be any, it must be extremely 

rarefied, perhaps as much so as that in the vacuum 

obtained in the receiver of our best air-pumps. 

Appearance of the Earth to Lunarians.— If there 

Wia L9 be any lunar inhabitants 

on the side toward us, the 

earth must present to 

them all the phases which 

their world exhibits to us, 

only in a reverse order. 

When we have a new 

moon, they have a full 

earth, a bright full-orbed 

moon fourteen times as 

large as ours. The lunar 

inhabitants upon the side 

opposite to us of course 

never see our earth, unless they take a journey to 




Appearance of the Earth a* seen from the 
Moon. 



THE MOON. 127 

the regions from whence it is visible, to behold this 
wonderful spectacle. Those living near the limbs of 
the disk might, however, on account of the liga- 
tions, get occasional glimpses of it near their hori- 
zon. 

The Earth-Shine. — For a few days before and after 
new moon, we may distinguish the outline of the 
unillumined portion of the moon. In England, it is 
popularly known as "the old moon in the new 
moon's arms." This reflection of the earth's rays 
must serve to keep the lunar nights quite light, even 
in new earth. 

Phases of the Moon. — The phases of the moon 
show conclusively that it is a dark body, which 
shines by reflecting the light it receives from the 
sun. Let us compare its various appearances with 
the positions indicated in the figure. 

(1.) We see the moon as a delicate crescent in the 
west just after sunset, as she emerges from the sun's 
rays at conjunction. She soon sets below the hori- 
zon. Half of the surface is illumined, but only a 
slender edge with the horns turned from the sun is 
visible to us. Each night the crescent broadens, the 
moon recedes about 13° further from the sun, and 
sets correspondingly later, until at quadrature half 
of the enlightened hemisphere is turned toward us, 
and the moon is said to be in her first quarter. 

(2.) The moon, continuing her eastern progress 
round the earth, becomes gibbous* in form, and, 
about the fifteenth day from new moon, reaches the 
point in the heavens directly opposite to that which 

* Gibbons means more than a half and less than the whole of a circle. 



128 



THE SOLAR SYSTEM. 



the sun occupies. She is then in opposition, the 
whole of the illumined side is turned toward us, and 
we have a full moon, She is on the meridian at mid- 



Fig. 50. 




Phases of the Moon. 



night, and so ; rises in the east as tho sun sets in the 
west, and vice versa. 



THE MOON. 129 

(3.) The moon, passing on in her orbit from oppo- 
sition, presents phases reversed from those of the 
second quarter. The proportion of the illumined 
side visible to us gradually decreases ; she becomes 
gibbous again ; rises nearly an hour later each even- 
ing, and in the morning lingers high in the western 
sky after sunrise. She now comes into quadrature, 
and is in her third quarter. 

(4). From the third quarter, the moon turns her 
enlightened side from us and decreases to the 
crescent form again ; as, however, the bright hemi- 
sphere constantly faces the sun, the horns are 
pointed toward the west. She is now seen as a 
bright crescent in the eastern sky just before sun- 
rise. At last, the illumined side is completely turned 
from us, and the moon herself, coming into conjunc- 
tion with the sun, is lost in his rays. To accomplish 
this journey through her orbit from new moon 
to new moon again, has required 29^ days — a lunar 
month. 

Moon Kuns High or Low. — All have, doubtless, 
noticed that, in the long nights of winter, the full 
moon is high in the heavens, and continues a long 
time above the horizon ; while in midsummer she is 
low, and remains a much shorter time above the 
horizon. This is a wise plan of the Creator, which is 
seen yet more clearly in the arctic regions. There, 
the moon, during the long summer day of six 
months, is above the horizon only her first and fourth 
quarters, when her light is least; but during the 
tedious winter night of eojial length, she is continu- 
ally above the horizon for her second and third 



130 THE SOLAR SYSTEM. 

quarters. Thus, in polar regions, the moon is never 
full by day, but is always full every month in the 
night. 

We can easily understand these phenomena when 
we remember that the new moon is in the same 
quarter with, and the full moon is in the opposite 
quarter from, the sun. When, therefore, the sun 
sinks low in the southern sky the full moon rises 
high, and when the sun rises high the full moon 
sinks low. 

Harvest Moon. — While the moon rises, on the 
average, 50 m. later each night, the exact time 
varies from less than half an hour to a full hour. 
Near the time of the autumnal equinox the moon, at 
her full, rises about sunset for a number of nights 
in succession. This produces a series of brilliant 
moonlight evenings. It is the time of harvest in 
England, and hence has there received the name of 
the Harvest Moon. In the following month (October), 
the same occurence takes place ; it is then termed the 
Hunter's Moon. 

The cause of this phenomenon lies in the fact that 
the moon's path is variously inclined to the horizon 
at different seasons of the year. * When, at the time 
of rising, the full moon is near the vernal equinox, 
the angle her path makes with the horizon is least, 
and when she is near the autumnal equinox it is 
greatest. In the former case, the moon, moving 
eastward each day about 13°, will descend but little 
below the horizon, and so for several successive 

* Besides this reason, we should remember that the motion of the moon is slowest 
a* apogee and fastest at perigee. (See note, \>. 302.) 



THE MOON. 



131 



Fig. 51. 



evenings will rise at about the same hour. In the 
latter, she will descend much further each day and 
thus will rise much later each night. The least pos- 
sible variation in the hour of rising is 17 minutes, — 
the greatest is 1 hour and 16 minutes. 

In Figure 51, let S represent the sun ; E, the earth ; 
M, the moon ; C F, the moon's path around the earth 
when the autumnal equinox is in the eastern horizon ; 
E D, when the vernal equinox is in the eastern hori- 
zon ; A M B S, the horizon ; and Md=Mb= 13°, the 
distance the moon 
moves each day. 
When passing 
along the path C 
F, the moon sinks 
below the horizon 
the distance a b, 
and when moving 
along the path E 
D, only the dis- 
tance c d. It is 
obvious that be- 
fore the moon can 
rise in the former 
case, the horizon 

must be depressed the distance a 6, and in the latter 
only c d ; and the moon will rise each evening cor- 
respondingly later in the one and earlier in the other. 

Cause of "Dry Moon," and "Wet Moon." — At new 
moon, when the bright crescent lies nearly perpen- 
dicular to the horizon, the moon is popularly called 
a wet moon, and when it is almost horizontal, the 




The Harvest Moon. 



132 THE SOLAR SYSTEM. 

moon is termed a dry moon. The cause of this 
change in the crescent is astronomical, and not 
meteorological. The form of the crescent has there- 
fore no connection with the weather. A little reflec- 
tion will show us that the horns, or cusps, of the 
new moon must point from the sun. As the ecliptic 
(from which the moon's path varies but slightly) is 
differently inclined to the horizon at various times 
of the year, this will give the crescent a different 
position with reference to the horizon (p. 29). 

Nodes. — The orbit of the moon is inclined to the 
ecliptic about 5°, the points where her path crosses 
it being termed nodes. The ascending node ( Q ) is 
the place where the moon crosses in coming above 
the ecliptic, or toward the north star ; the descending 
node ( g ) is where it passes below the ecliptic. The 
imaginary line connecting these two points is called 
the "line of the nodes." 

Occultation. — The moon, in the course of her 
monthly journey round the earth, frequently passes 
in front of the stars or planets, which disappear on 
one side of her disk and reappear on the other. This 
is termed an occultation, and is of practical use in 
determining the difference of longitude between 
various places on the earth. 

Lunar Seasons; Day and Night, Etc. — As the 
moon's axis is so nearly perpendicular to her orbit, 
she cannot have any change of seasons. During 
nearly fifteen of our days, the sun pours down his 
rays unmitigated by any atmosphere to temper them. 
To this long, torrid day succeeds a night of equal 
length and polar cold. 







133 



Ideal Landscape on the Moon. 



134 THE SOLAR SYSTEM. 

How strange the lunar appearance would be to us ! 
The disk of the sun seems sharp and distinct. The 
sky is black and overspread with stars even at mid- 
day. There is no twilight, for the sun bursts 
instantly into day, and, after a fortnight's glare, as 
suddenly gives place to night ; no air to conduct 
sound ; no clouds ; no winds ; no rainbow ; no blue 
sky ; no. gorgeous tinting of the heavens at sunrise 
and sunset ; no delicate shading ; no soft blending 
of colors, but only sharp outlines of sun and shade. * 

The nights of the visible hemisphere must be 
brilliantly illuminated by the earth, whose phases 
" serve well as a clock — a dial all but fixed in the 
same part of the heavens, like an immense lamp, 
behind which the stars slowly defile along the black 
sky." 

Telescopic Features. — Even with the naked eye, 
we see on the moon's surface bright spots (the sum- 
mits of lofty mountains, gilded by the first rays of 
the sun), and darker portions — low plains yet lying 
in comparative shadow. The telescope reveals to 
us a region torn and shattered by fearful though 
now extinct f volcanic action. Everywhere the 



* The moon is a fossil world, an ancient cinder, a ruined habitation perpetuated only 
to admonish the earth of her own impending fate, and to teach her occupants that another 
home must be provided, which frost and decay can never invade. The moon was once 
the seat of all the varied and intense activities that now characterize the surface of our 
earth. At one time its physical condition was like that of the parent earth from which 
it had just been separated : but, being smaller, it cooled faster, and its geologic periods 
were correspondingly shorter. Its life-age was perhaps reached while the earth was yet 
glowing. — Read Winchell's Geology of the Stars. 

t Several distinguished astronomers assert, however, that the crater Linnaeus has 
undergone noticeable transformations. Its sides seem to have fallen in, and the interior 
to have become filled up, as if by a new eruption. It is said to present an appearance 
similar to that of the Sea of Serenity. Other marked changes are said to have been 
discovered from time to time, on the moon's surface, but they are not generally ac- 



THE MOON. 



135 




Telescopic View of the Moon, 



136 



THE SOLAR SYSTEM. 



crust is pierced by craters, whose irregular edges 
and rents testify to the convulsions our satellite has 
undergone. 

Mountains,— The heights of more than 1,000 of 
the lunar mountains have been measured, some of 
which exceed 25,000 feet. When the sun's rays strike 
one of these mountains obliquely, the shadow is as 
distinctly perceived as that of an upright staff when 
placed opposite the sun. Some of the elevations are 
insulated peaks that shoot up from the center of 
circular plains ; others are mountain ranges extend- 



Fig. 5U. 




ing hundreds of miles. Most of the lunar heights 
have received names of men distinguished in science. 
Thus we find Plato, Aristarchus, Copernicus,* Kepler, 

credited. For an interesting discussion of this subject, read a chapter entitled "A New 
Crater in the Moon," in Proctor's Poetry of Astronomy. 

* This is one of the grandest of the lunar craters. It is situated on the tip of the nose 
of the " Man in the Moon." Its diameter is forty-six miles, and its encircling rampart 
rises 12,000 feet above the interior plateau, in the midst of which stands a group of 
cones, one 2,400 feet in height, 



THE MOON. 137 

and Newton, associated, however, with the Apen- 
nines, Carpathians, etc. 

Gray Plains, or Seas. — These are analogous to 
our prairies. They were formerly supposed to be 
sheets of water, but they exhibit the uneven ap- 
pearance of a plain, instead of the regular curve of a 
sea. The former names have been retained, and we 
find on lunar maps the Sea of Tranquillity, the Sea 
of Nectar, Sea of Serenity, etc. 

Rills, Luminous Bands. — The latter are long, 
bright streaks, irregular in outline and extent, which 
radiate in every direction from Tycho, Kepler, and 
other mountains ; the former are similar, but are 
sunken, and have sloping sides, and were at first 
thought to be ancient river-beds. Their nature is a 
mystery. 

Craters constitute the most curious feature of the 
lunar landscape. They are of volcanic origin, and 
usually consist of a cup-like basin, with a conical 
elevation in the center. Some of the craters have a 
diameter of over 100 miles, and are great walled plains, 
sunk so far behind huge, volcanic ramparts that the 
lofty wall surrounding an observer at the center 
would be beyond his horizon. Other craters are 
deep and narrow, — as Newton, which is said to be 
about four miles in depth, — so that neither earth nor 
sun is ever visible from a great part of the bottom. 
The appearance of these craters is strikingly shown 
in the accompanying view (Fig. 53) of the region to 
the southeast of Tycho. 



138 



THE SOLAR SYSTEM. 



ECLIPSES. 

Eclipse of the Sun. — If the moon should pass 
through either node at or near the time of conjunc- 
tion, or new moon, she would necessarily come be- 
tween the earth and the sun, for the three bodies 
are then in the same straight line. This would cause 




<ry of a Total and a Partial Eclipse of the Sun. 



an eclipse of the sun. If the moon's orbit were in 
the same plane as the ecliptic, an eclipse of the sun 
would occur at every new moon i but as the orbit is 
inclined, it can occur only at or near a node. 



" ■ — Mggjji'- m x 

z -^iBH^HBHii lisp * 



Theory of an Annular Eclipse of the Sun. 



The eclipse may be partial, total, or annular. 
-In Fig. 55, we see where the dark shadow (umbra) 



ECLIPSES. 139 

of the moon falls on the earth and obscures the 
entire body of the sun. To the persons within that 
region, there is a total eclipse; the breadth of this 
space is not large, averaging only 140 miles. Be- 
yond this umbra, there is a lighter shadow, penum- 
bra, (pene, almost ; umbra, a shadow), where only a 
portion of the sun's disk is obscured. Within this 
region, there is a partial eclipse. To those persons 
living north of the equator and of the umbra, the 
eclipse passes over the lower limb of the sun ; to 
those south of the umbra, it passes over the upper 
limb. * When the eclipse occurs exactly at the node, 
it is said to be central. If the eclipse takes place 
when the moon is at apogee, her apparent diameter 
is less than that of the sun ; as a consequence, her 
disk does not cover the disk of the sun, and the vis- 
ible portions of that luminary appear in the form of 
a ring (annulus) ; hence there is an annular eclipse 
in all those places comprised within the limits of the 
cone of shadow prolonged to the earth. 

General facts concerning a solar eclipse. — The 
following data may guide in understanding the 
phenomena of solar eclipses. 

(1.) The moon must be new. 

(2.) She must be at or near a node. 

(3.) When her distance from the earth is less than 
the length of her shadow, the eclipse will be total or 
partial. 

(4.) When her distance is greater than the length 
of her shadow, the eclipse will be annular or partial. 

* South of the equator the reverse of these phenomena would happen. 



140 THE SOL AH SYSTEM, 

(5.) There can be no eclipse at those "places where 
the sun himself is invisible. 

(6.) An eclipse is not visible over the whole illu- 
mined side of the earth. As the moon's diameter is 
less than that of the earth, her cone of shadow is too 
small to enshroud the entire globe, so that the region 
in which it is total cannot exceed 180 miles in 
breadth. As, however, the earth is constantly rotat- 
ing on its axis during the duration of the eclipse, 
the shadow may travel over a large surface. 

(7.) If the moon's shadow fall upon the earth when 
she is nearing her ascending node, it will sweep 



across the south polar regions : if, when nearing her 
descending node, it will graze the earth near the 
north pole. The nearer a node a conjunction occurs, 
the nearer the equatorial regions the shadow will 
strike. 

(8.) At the equator, the longest possible duration 
of a total solar eclipse is about eight minutes ; of an 
annular, twelve minutes. One reason of the greater 
length of the latter is, that then the moon is in 
apogee, when she always moves slower than in 
perigee. The duration of total obscuration is great- 
est when the moon is in perigee and the sun in 
apogee ; for then the apparent size of the moon is 
greatest, and that of the sun is least; 



ECLIPSES. 



141 



(9.) There cannot be more than five nor less than 
two solar eclipses per year. A total or an annular 
eclipse, in its recurrence at any place, is exceedingly 
rare. There has been (according to Halley) only one 
total eclipse visible at London since 1140. 

(10.) A solar eclipse comes on the western limb, or 
edge of the sun, and passes off on the eastern. 

(11.) The disk of the sun is divided into twelve 
digits, and the amount of the eclipse is estimated by 
the number of digits which it covers. Thus an 
eclipse of six digits is one in which half the diam- 
eter of the disk is concealed. 3|. 

Curious phenomena attend a total eclipse. Aroffncl 
the sun is seen a 
beautiful corona, 
or halo of light, 
like that which 
painters give to the 
head of the Virgin 
Mary. Flames of 
a rose-red color 
play around the 
disk of the moon. 
When only a mere 
crescent of the sun 
is visible, it seems 
to resolve itself 
into bright spots 
interspersed with 
dark spaces, having the appearance of a string of 
glittering beads (Baily's Beads). 

The attendant circumstances of a total : eclipse 




Eclipse of 1858. 



142 



TltE SALAR SYSTEM. 



are of a peculiarly impressive character. The dark- 
ness is so dense that the brighter stars and planets 
are seen, birds cease their songs and fly to their 
nests, flowers close, and the face of nature assumes 
an unearthly, cadaverous hue, while a sudden fall 

of the temperature 

Fig. 59. , , 

causes the air to 
feel damp, and the 
grass to be wet as 
if from excessive 
dew. Orange, yel- 
low, and copper 
tints give objects 
a strange appear- 
ance. "Men look 
at each other, and 
behold, as it were, 
corpses. " 

The ancients re- 
garded a total 
eclipse with feel- 
ings of indescribable terror, as an indication of the 
anger of an offended Deity, or the presage of some 
impending calamity.* Even now, when the causes 

* William of Malmesbury thus connects the eclipse of August 2, 1133, with Henry I., 
who left England on that day, never to return alive : " The elements manifested their 
sorrows at this great man's last departure. For the sun on that day, at the 6th hour, 
shrouded his glorious face, as the poets say, in hideous darkness, agitating the hearts 
of men by an eclipse : and on the 6th day of the week, early in the morning, there was 
so great an earthquake that the ground appeared suddenly to sink down ; an horrid 
noise being first heard beneath the surface." 

The same writer, speaking of the total eclipse of March 20, 1140, says : " During this 
year, in Lent, on the 13th of the kalends of April, at the 9th hour of the 4th day of the 
week, there was an eclipse, throughout England, as I have heard. "With us, indeed,; and 
with all our neighbours, the obscuration of the Sun also was so remarkable, that persons 
sitting at table, as it then happened almost every where, for it was Lent, at first feared 




Annular Eclipse of 1835, shovnng Baily's Beads. 




Corona seen in 1871. 



are fully understood, and the time of the eclipse can 
be predicted within the fraction of a second, the 



that Chaos was come again : afterwards learning the cause, they went out and beheld 
the stars around the Sun. It was thought and said by many, not untruly, that the king 
(Stephen) would not continue a year in the government." 

Columbus made use of an approaching eclipse of the moon, which took place March 1, 
1504, to relieve his fleet, then in great distress from want of supplies. As a punishment 
to the islanders of Jamaica, who refused to assist him, he threatened to deprive them 
of the light of the moon. At first they were indifferent to his threats, but " when the 
eclipse actually commenced, the barbarians vied with each other in the production of 
the necessary supplies for the Spanish fleet." 

Among the Hindoos a singular custom is said to exist. When, during a solar eclipse, 
the black disk of our satellite begins slowly to advance over the sun, the natives believe 
that some terrific monster is gradually devouring it. Thereupon they beat gongs, and 
rend the air with screams of terror and shouts of vengeance. For a time their frantic 



144 THE SOLAR SYSTEM. 

change from broad daylight to almost instantaneous 
gloom is overwhelming, and inspires with awe even 
the most careless observer. (See note, p. 303.) 

The Saros. — The nodes of the moon's orbit are con- 
stantly moving backward. They complete a revolu- 
tion around the ecliptic in about 18£ years. Now 
the moon makes 223 synodic revolutions in 18 years 
and 10 days ; the sun makes 19 revolutions with re- 
gard to the lunar nodes in about the same time. 
Hence, in that period, the sun, the moon, and the 
nodes will be in nearly the same relative position. 
If, then, we reckon 18 years and 10 days from any 
eclipse, we shall find the time of its repetition. 

This method was discovered, it is said, by the Chal- 
deans. The ancients were enabled, by this means, 
to predict eclipses, but it is considered too inaccurate 
by modern astronomers. 

Metonic Cycle. — The Metonic Cycle (sometimes 
confounded with the Saros) was not used for fore- 
telling eclipses, but for ascertaining the age of the 
moon at a given period. It consists of nineteen 
tropical years,* during which time there are 235 new 
moons.;: so that, at the end of /this period, the new 
moons will recur at seasons of the year correspond- 
ing to those of the preceding cycle. By registering, 
therefore, the exact days of any cycle at which the 

efforts seem futile and the eclipse still progresses. At length, however, the increasing 
"uproar reaches the voracious monster ; he appears to pause, and then, like a fish reject- 
ing a nearly swallowed bait, gradually disgorges the fiery mouthful. When the sun is 
quite clear of the great dragon's mouth, a shout of joy is raised, and the-poor natives 
disperse, delighted to think that they have so successfully relieved their deity from his 
impending peril. 

* A tropical year is the interval between two successive returns of the sun to the 
vernal equinox. • ,:..... . ,, . ;: .5 .. ,. 



PLATE m. 




eclipses. - -.: 145 

new and full moons occur, suck a calendar shows on 
what days these events will happen in succeeding 
cycles. : 

Since the appointment of games, feasts, and fasts 
has been made very extensively, both in ancient and 
modern times, according to new or full moons, such 
a calendar becomes very convenient for finding the 
day on which the required new or full moon takes 
place. Thus, if a festival were decreed to be held in 
any given year on the day of the first full moon after 
the vernal equinox : find what year it is of the lunar 
cycle, then refer to the corresponding year of the 
preceding cycle, and the day will be the same. The 
Golden Number, a term still used in our almanacs, 
denotes the year of the lunar cycle. Four is the 
golden number for 1884. 




Eclipse of the Moon. 



An Eclipse of the Moon is caused by the passing 
of the moon into the shadow of the earth, and hence 
can take place only at full moon — opposition. As 
the. moon's orbit is inclined to the ecliptic, her path 
is partly above and partly below the earth's shadow ; 
thus an eclipse of the moon can take place only at 
or near one of the nodes. In Fig. 62, the umbra is 
represented by the space between the lines K c and 
7 



146 THE SOLAR SYSTEM. 

I b ; outside of this is the penumbra, where the earth 
cuts off the light of only a portion of the sun. The 
moon enters the penumbra of the earth at a, — this is 
termed her first contact with the penumbra ; next she 
encounters the dark shadow of the earth at b, — this is 
called the first contact with the umbra ; she then 
emerges from the umbra at c, — the second contact 
with the umbra; finally, she touches the outer edge 
of the penumbra at d, — the second contact with the 
penumbra. Since the earth is so much larger than 
the moon, the eclipse can never be annular; as, 
however, the eclipse may occur a little above or be- 
low the node, the moon may only partly enter the 
earth's shadow, either on its upper or lower limb. 
From the first to the last contact with the penumbra, 
five hours and a half may elapse. 

Total eclipses of the moon are rarer events than 
those of the sun, since the lunar ecliptic limit is only 
about 12° ; yet they are more frequently seen by us, 
(1) because each one is visible over the entire unil- 
lumined hemisphere of the earth, and also (2) be- 
cause by the diurnal rotation during the long dura- 
tion of the eclipse, large areas may be brought 
within its limits. So it will happen that while the 
inhabitants of one district witness the eclipse through- 
out its continuance, those of other regions merely 
see its beginning, and others only its termination. 

The moon does not completely disappear even in 
total eclipses. The cause of this lies in the refraction 
of the solar rays in traversing the lower strata of 
the earth's atmosphere ; they are analyzed, and 
purple our moon with the tints of sunset, The 



THE TIDES. 



147 



amount of refraction and the color depend upon the 
state of the air at the time. 



THE TIDES. 

Description. — Twice a day, at intervals of about 
twelve hours and twenty-five minutes, the water be- 
gins to set in from the ocean, beating the pebbles 
and the foot of the rocky shore, and dashing its 
spray high in air. For about six hours, it climbs far 
up on the beach, flooding the low lands and trans- 
forming creeks into rivers. The instant of high-water 
or flood-tide being reached, the water begins to 
descend, and the ebb succeeds the flow. The water, 
however, falls somewhat slower than it rises. 



Fig. 63. 




The Tides are Caused by a great wave, which, 
raised by the moon's attraction, follows her in her 
course around the earth.* The sun, also, aids some- 

* Prof. Ball, Royal Astronomer of Ireland, claims that once the moon was nearer the 
earth than now ; the day and the month were equal, each three hours long. At 40,000 
miles distance, the moon was a greater tide producer by 216 times. As the moon receded 
from the earth, both revolved more slowly. At the present time, 27 earth rotations 
equal one moon rotation. This has remained, and will remain, sensibly true, for thou- 
sands of years. But the friction of the tides will, in the far future, lengthen the day to 
equal 57 of our present days,— a condition that will then last for ages, 



148 THE SOLAR SYSTEM. 

what in producing this effect ; but as the moon is 400 
times nearer the earth, her influence is far greater.* 

As the waters are free to yield to the attraction of 
the moon, she draws them away from C and D and 
they become heaped up at A. The earth, being 
nearer the moon than the waters on the opposite 
side, is more strongly attracted, and so, being drawn 
away from them, they are left heaped up at B. As 
the result, high-water is produced at A by the water 
being pulled from the earth, and at B by the earth 
being pulled from the water. 

The influence of the moon requires a little time to 
produce its full effect ; hence high-water does not 
occur at any place when the moon is on the me- 
ridian, but a few hours after. As the moon rises 
about fifty minutes later each day, there is a corre- 
sponding difference in the time of high-water. 
While, however, the lunar tide- wave thus lags about 
fifty minutes every day, the solar tide occurs uni- 
formly at the same time. They therefore steadily 
separate from each other. At one time, they coin- 
cide, and high-water is the sum of the lunar and 
solar tides ; at other times, high-water of the solar 
tide and low-water of the lunar tide occur simul- 
taneously, and high-water is then the difference be- 
tween the lunar and solar tides, f 

* The whole attraction of the moon is only j]^ that of the sun : yet her influence in 
producing the tides and precession is greater, because that depends not upon the entire 
attraction either exerts, but upon the difference between their attraction upon the 
earth's center and upon the earth's nearest surface. For the moon, on account of her 
nearness, the proportion of the distance of these parts is treble that of the sun, and 
hence her greater effect. 

t We should bear in mind the philosophical truth, that the tide in the open sea does 
not consist of a progressive movement of tlie water itself, but only of the form of the 
wave.— Physics, p. 101. 



THE TIDES. 



149 



Causes that modify the tides. — At new and at 
full nioon (the syzygies) the sun acts with the moon 
(Fig. 63) in elevating the waters ; this produces the 
highest, or Spring-tide. In quadrature (Fig. 64), the 
sun tends to diminish the height of the water : this 
is called Neap-tide. When the moon is in perigee, 
her attraction is stronger ; hence the flood-tide is 
higher, and the ebb-tide is lower than at other times. 




Neap Tide. 



This remark applies also to the sun. The height of 
the tide also varies with the declination of the sun 
and the moon, — the highest or equinoctial tides tak- 
ing place at the equinoxes, if, when the sun is over 
the equator, the moon also happens to be very near 
it : the lowest occur at the solstices. The force and 
the direction of the winds, the shape of the coast, 
and the depth of the sea greatly complicate the ex- 
planation of local tides. 

Height of the tide at different places. — In the 
open sea, the tide is hardly noticeable, the water 
sometimes rising not higher than a foot ; but where 
the wave breaks on the shore, or is forced up into 



150 THE SOLAR SYSTEM. 

bays or narrow channels, it is very conspicuous. 
The difference between ebb and flood neap-tide at 
New York is over three feet, and that of spring-tide 
over five feet ; while at Boston it is nearly double 
this amount. A headland jutting out into the ocean 
will diminish the tide ; as, for instance, off Cape 
Florida, where the average height is only one and a 
half feet. A deep bay opening up into the land like 
a funnel will converge the wave, as at the Bay 
of Fundy, where it rolls in, a great, roaring wall 
of water sixty feet high, frequently overtaking and 
sweeping off men and animals.* The tide sets up 
against the current of rivers, and often entirely 
changes their character ; for exam/pie, the Avon 
at Bristol is a shallow ditch, but at flood-tide it 
becomes a deep channel navigable by the largest 
Indiamen. 



V. MARS. 

The god of war. .Sign, 6 , shield and spear. 

Description. — Passing outward in our survey of the 
solar system, we next meet with Mars. This is the 
first of the superior planets, and the one most like 
the earth. It appears to the naked eye as a bright 
red star, rarely scintillating, and shining with a 
steady light, which distinguishes it from the fixed 



The tide-wave ascends the Hudson River at about the same speed as the steam- 
boats ; at Albany it reaches a height of a little over two feet. 



MARS. 



151 



stars.* At conjunction its apparent diameter is only 
about 4"; but once in about two years it comes into 
opposition with the sun, when its diameter may in- 
crease to 30". At intervals of nearly 15 years, this 
occurs when the planet is in perihelion and the earth 
in aphelion. Mars then shines with a brilliancy 
rivalling that of Jupiter himself, f 





Diameter of Mars at Extreme, Mean, and Least Distances. 

Motion in Space. — Mars revolves around the Sun 
at a mean distance of about 141,000,000 miles. Its 
orbit is sufficiently flattened to bring it at perihelion 
26,000,000 miles nearer that luminary than when in 
aphelion. Its motion varies in different portions of 
its orbit, but the average velocity is about fifteen 
miles per second. The Martian day is 37 min. longer 
than ours, and the year contains about 668 Martian 
days, equal to 687 terrestrial days (nearly two years). 

Distance from Earth. — When in opposition, the 



* Its ruddy appearance has led to its being celebrated among all nations. The Jews 
,ve it the appellation of "blazing," and it bore in other languages a similar name, 
t The next favorable opposition will occur in 1892. 



152 THE SOLAR SYSTEM. 

distance of Mars is (like that of all the superior 
planets) the difference between the distance of the 
planet and that of the earth from the Sun : at con- 
junction, it is the sum of these distances. If the 
orbits were circular, these distances would be the 
same at every revolution. The elliptical figure, how- 
ever, occasions much variation. Thus, if Mars, at 
opposition, be in perihelion while the earth is in aphe- 
lion, it is removed from us about 34,000,000 miles. 

Dimensions. — The diameter of Mars is nearly 4,200 
miles. * Its volume is about } and its density | that 
of the earth. A stone let fall on its surface would 
fall six feet the first second. It is somewhat flat- 
tened at the poles, and bulged at the equator, like our 
globe. 

Seasons. — The light and heat of the sun at Mars 
are less than one-half that which we enjoy. Its axis 
is inclined about 27°, therefore its zones and seasons 
do not differ materially from our own : its days, 
also, as we have seen, are of nearly the same length. 
Since, however, its year is equal to nearly two of 
our years, the seasons are lengthened in proportion. 

There must be a considerable difference between 
the temperature of its northern and southern hemi- 
spheres, as the former has its summer when 26,000,000 
miles further from the sun than the latter : an in- 
creased length of 76 days may, however, be sufficient 

* Some authors place the diameter of Mars at about 5,000 miles. There is, also, a 
discrepancy as to the other data of this planet. Prof. Hall, as the result of his 
observations, gives the density = .776 ; force of gravity = .37 ; fall of a body, 1st 
sec. — (3 feet. With the discovery of the satellites we have now the means of securing 
exact results. The difficulty of observation, however, is shown from the fact that " the 
light which falls upon the earth from one of these moons is about what a man's hand on 
which the sun shines at Washington would reflect to Boston." 



MARS. 



153 



compensation. It has an atmosphere like our own, 
loaded with clouds. 

Mars has two moons. * Our earth and its moon pre- 
sent in the Martian evening sky a beautiful pair of 
planets, constantly remaining in close proximity to 
each other, and exhibiting 
all the phases which Mer- 
cury and Venus present to 
us. 

Telescopic Features. — Un- 
der the telescope, Mars ex- 
hibits slight phases. Its 
surface is covered with red- 
dish spots, which are be- 
lieved to be continents, f 
Other portions, of a green- 
ish tint, are considered to 
be bodies of water. The proportion of land to water 
on the earth is reversed in Mars. "Here every con- 
tinent is an island ; there every sea is a lake : but 




View of Mm 



* The satellites of Mars were discovered in August, 1877, by Prof. Hall of the Naval 
Observatory, Washington. The outer one revolves about the planet in 30 hr. 18 min., . 
at a distance of about 12,300 miles ; and the inner one in 7 hr. 40 min., at a distance of 
3,600 miles (less than that of remote cities on our own continent). The inner moon 
moves so much faster than the rotation of Mars that to an inhabitant of that planet, the 
moon would seem to rise in the west and set in the east, passing through all the phases 
of our moon during a single night. The moons have been named Deimos and Phobus, 
or Dread and Terror— the sons of Mars. The diameter of these little globes is probably 
less than 15 miles. For an amusing description of such a world, read " Living in Dread 
and Terror," a chapter in Proctor's " Poetry of Astronomy." 

t So carefully has the surface of this planet been studied, that a globe of Mars has 
been prepared which is said to be in some respects more perfect than any globe of the 
earth. The different bodies of land and water have been named after distinguished 
astronomers. A characteristic feature of the seas is the long, narrow channels. Schi- 
aparelli, the Italian astronomer, claims to have discovered a number of singular dark 
lines, now known as " canals." They seem to connect different bodies of water, and, 
though without sufficient reason, have been by some considered as the work of the Mar- 
tian inhabitants. 



154 THE SOLAR SYSTEM. 

these, like our own continents, are chiefly confined 
to one hemisphere, so that the habitable area of the 
two globes may not differ so much as the size of the 
planets. " 

The ruddy color is thought by Herschel to be due 
to an ochery tinge in the soil ; by others it is attrib- 
uted to peculiarities of the atmosphere and clouds. 
Lambert suggests that the color of the vegetation on 
Mars may be red instead of green. There are con- 
stant changes going on in the brightness of the disk, 
owing, it is supposed, to the variation of the clouds 
of vapor in its atmosphere. No mountains have yet 
been discovered. 

In the vicinity of the poles are brilliant white 
spots, which are considered to be masses of snow. 
The "snow zones" apparently melt and recede with 
the return of summer in each hemisphere, and in- 
crease on the approach of winter. We can thus from 
the earth watch the formation of polar ice and the 
fall of snow, — in fact, the changes of the seasons — on 
the surface of a neighboring planet. 



VI. THE MINOR PLANETS. 

Discovery. — Beyond Mars there is a wide interval 
that was not filled until the present century. The 
bold, imaginative Kepler conjectured that there was 
a planet in this space. This supposition was cor- 
roborated by Titius's discovery of what has since 
been known as 

Bode's Law. — Take the numbers 0, 3, 6, 12, 24, -18, 



THE MINOR PLANETS, OR ASTEROIDS. 155 

96, 192, 384, each of which, after the second, is double 
the preceding one. If we add 4 to each of these 
numbers, we form a new series : 

4, 7, 10, 16, 28, 52, 100, 196, 388. 

At the time this law was discovered, these numbers 
represented very nearly the proportionate distance 
from the sun of the planets then known, taking* the 
earth's distance as ten, except that there was a blank 
opposite 28. This naturally led to inquiry, and a 
systematic effort to solve the mystery. * 

On the 1st day of January, 1801, the nineteenth 
century was inaugurated by Piazzi's discovery of 
the small planet Ceres, at almost the exact distance 
necessary to fill the gap in Bode's series. The an- 
nouncement of other new planets soon followed, 
until now (1884) there are two hundred and thirty- 
seven, with a probability of more being found. In- 
deed, Leverrier has calculated that there may be 
perhaps 150,000 in all. 

Description. — These minor worlds, or "pocket 
planets," as Herschel styled them, are diminutive 
indeed. The largest of them is Yesta, which shines 
at times as a star of the 6th magnitude, and can then 
be seen with the naked eye. f Those recently discov- 

* It is a curious fact that the discovery of Ceres should have been made by an out- 
sider, as Piazzi did not belong to the society of 24 astronomers then searching for the 
planet. The publication of Bode's law had little to do with the result. In fact, the 
direct cause was an error of the press in putting an extra star in Wollaston's Catalogue, 
and while Piazzi was looking for this star he found Ceres. 

t The small size of the disks of the minor planets defies exact measurement. New- 
comb makes Ceres and Vesta the largest of the group, with diameters between 200 and 
400 miles. Echo has been assigned a diameter of 17 miles, or not far from the size of the 
miniature moons of Mars. Several of these little worlds have been found but to be lost 
again ; while the mere labor of tracing the movements of so many tiny globes already 
surpasses the probable worth of the results. 



156 THE SOLAR SYSTEM. 

ered are so small that it is difficult to decide which 
is the smallest. A good walker could easily make the 
tour of one in a day ; a prairie farmer would need to 
pre-empt a whole such world for a cornfield. "A 
man placed on one of these tiny globes could leap 
60 feet high, and, in his descent, would sustain no 
greater shock than he does on the earth from jump- 
ing or leaping a yard. " These planets revolve around 
the sun in regular orbits, comprising a zone about 
100,000,000 miles in width. Their paths are variously 
inclined to the ecliptic ; Massalia's is only 41', while 
that of Pallas rises 34°. 

Origin. — A conjecture concerning the origin of these 
bodies is, that they are the fragments of a large 
planet that, in a remote antiquity, was shivered to 
pieces by some terrible catastrophe. " One fact 
seems above all others to confirm the idea of an inti- 
mate relation between these planets. It is this : if 
their orbits consisted of solid rings, they would be 
found so entangled that it would be possible, by 
taking up any one at random, to lift all the rest." 
The more probable view is given under the " Nebular 
Hypothesis." 

Names and Signs.— Ceres, the first discovered, re- 
ceived the symbol ? , a sickle, as that goddess was 
supposed to preside over harvests. Pallas, the 
second, named from the goddess of wisdom and sci- 
entific warfare, obtained the sign $ , the head of a 
spear. Of late, a simple circle with the number 
inclosed has been adopted ; thus © represents Ceres, 
is the sign of Pallas. 



JUPITER. 157 



VII. JUPITER. 

The king of the gods. Sign %, a hieroglyphic representation of an eagle, 
"the bird of Jove." 

Description. — From the smallest members of the 
solar system we now pass to the largest planet — the 
colossal Jupiter. Its peculiar splendor and brilliancy 
distinguish it from the fixed stars, and vie even with 
the lustre of Venus. It is one of the G.ve planets dis- 
covered in primitive ages. * 

Motion in Space. — Jupiter revolves about the sun 
at a mean distance of about 483,000,000 miles. His 
movement among the fixed stars is slow and majestic, 
comporting well with his vast dimensions and the 
dignity conferred by four attendant worlds. He 
advances through the zodiac at the rate of one 
sign yearly ; so that if we locate the planet 
now, a year hence we shall find it equally advanced 
in the next sign. Yet slowly as he seems to travel 
through the heavens, he is bowling along through 
space at the enormous speed of nearly 500 miles per 
minute. The Jovian day is equal to only about ten 
of our hours, while the year is lengthened to about 
12 of our years, comprising near 10,000 of his days. 

Distance from Earth. — Once in thirteen months 
Jupiter is in opposition, and his distance from the 
earth is measured by the difference of the distances 
of the two bodies from the sun. At the expiration 

■' In those early times, Jupiter was supposed to be the cause of storm and tempest. 
Pliny thought that lightning owed its origin to this planet. An old almanac of 1368, 
foretelling the harmless condition of Jupiter for a certain month, says, " Jubit es hote ■ 
and moyste and does weel til al thynges and noyes nothing." 



158 



THE SOLAR SYSTEM. 



of half this time he is in conjunction, and his dis- 
tance from us is measured by the sum of these 
distances. 

Dimensions. — The diameter of this planet is about 
90,000 miles. Its volume is 1.400 times that of the 
earth, and much ex- 
ceeds that of all the 
other planets com- 
bined. Seen at the 
distance of the moon . 
this immense globe 
would embrace 1,000 
times the space of the 
full moon. Its den- 
sity is only one-quar- 
ter that of the earth ; 
moreover, its rapid ro- 
tation upon its axis, 
whereby a particle on 
the equator revolves with a velocity of 473 miles per 
minute against the earth's 17 miles per minute, must 
produce a powerful centrifugal force which materi- 
ally diminishes the weight of objects near its 
equator. Consequently, a stone let fall on Jupiter 
would pass through only about 42 feet the first 
second. As a result of this rapid rotation, the 
planet is one of the most flattened of any in the 
solar system, the equatorial diameter exceeding the 
polar by 5,000 miles. 

Seasons. — As the axis of Jupiter is but slightly in- 
clined from a perpendicular to the plane of its orbit, 
there is little difference in the length of his days and 




View of Jupiter. 



JUPITEK. 159 

nights, which are each of about five-hours duration. 
At the poles, the sun is visible for nearly six years, 
and then remains set for the same length of time. 
The seasons are but slightly varied. Summer reigns 
near the equator, while the temperate regions enjoy 
perpetual spring. The light and heat of the sun are 
only 2T of what we receive ; yet peculiarities of soil 
or atmosphere may compensate this difference. The 
evening sky on Jupiter must be magnificent ; besides 
the glittering stars which adorn our heavens, four 
moons, waxing and waning, each with its diverse 
phase, illuminate his night. All the starry exhi- 
bition sweeps through the sky in five hours. 

Telescopic Features. — Jupiter's Moons. — Through 
the telescope* Jupiter presents a beautiful Copernican 
system in miniature. Four small stars— moons — ac- 
company him in his twelve-yearly revolutions. From 
hour to hour their positions vary, and they seem to 
oscillate from one side to the other of the planet. At 
one time, there will be two on each side ; and again, 
three on one side, while the remaining star is left 
alone. They are also frequently found to disappear, 
one, two, or even three at a time, and, more rarely, 
all four at once. 

These moons are called by the ordinal numbers, 
reckoning outward from the planet. With an ordi- 
nary glass, there is nothing to distinguish them 
from small stars. The Illrd., being the largest and 

* There are well-authenticated instances on record of their having been seen by the 
naked eye. Among others, the following singular case is mentioned. "Wrangle, the 
celebrated Russian traveler, states that, when in Siberia, he once met a hunter, who 
said, pointing to Jupiter, " I have just seen that star swallow a small one and then 
vomit it up again." 



160 



THE SOLAR SYSTEM. 



brightest, will generally be identified the most easily. 
The 1st. satellite appears to the inhabitants of the 
planet almost as large as our moon to us ; the Ilnd., 
and Illrd., about half as large. 



SATELLITES OF JUPITER. 



I. Io 

II. Europa . . . 

III. Ganymede. 

IV. Callisto... 



Mean distance 
from Jupiter. 



267,380 

425,156 

67S.393 

1,192,823 



2,352 in. 

2,009 " 
3,436 " 
2,929 " 



Density. 
Water as 1, 



1.12 

2.14 



Sidereal period. 



1 18 23 
3 13 4 



It is noticeable that here are four satellites revolv- 
ing about Jupiter, one of them larger than the 
planet Mercury, and each surpassing in size the 
minor planets between Mars and Jupiter. The 
moons are not only distinguised by their various 
dimensions, but also by the variety of their color. 
The 1st. and Ilnd. have a bluish tint, the Illrd. a 
yellow, and the IVth. a reddish shade. The space 
occupied by this miniature system is about two and 
a half million miles in diameter. 

Eclipse of the Moons. — Jupiter, like all celestial 
bodies not self-luminous, casts into space a cone of 
shade. The 1st., Ilnd., and Illrd. satellites revolve 
in orbits but very little inclined to the plane of the 
planet's orbit. During each revolution, they pass 
between the Sun and Jupiter, producing a solar 
eclipse ; and also, by passing through the shadow of 
the planet itself, cause to themselves an eclipse 
of the sun, and to Jupiter an eclipse of a moon. 
The IVth. moon passes through a path more in- 
clined, and therefore its eclipses are less frequent ; 



JUPITER. 



161 



instead of being fully eclipsed, it sometimes just 
grazes the shadow. Through a telescope, we can dis- 
tinctly watch the disappearance, or immersion, of the 




Eclipses and Occultations of Jupiter's Moons. 



satellites in the planet's shadow, their reappearance, 
or emersion, and also the transits of their shadows 
as round black dots moving across the disk of Jupiter. 



162 



THE SOLAR SYSTEM. 



In Fig. 68, we see various positions of the moons : 
the 1st. is eclipsed ; the Ilnd. is passing across the 
disk of the planet on which its shadow is also 
thrown ; the Illrd. is just behind the planet, and so 
occulted or concealed, while it has not yet entered 
the shadow; the IVth. is in view from the earth. 

These satellites revolve with great rapidity, as is 
necessary in order to overcome the superior attrac- 
tion of the planet and prevent their being drawn to 
its surface. The 1st. goes through all its phases in 
1J days ; the IVth., in less than twenty days. A 
spectator on Jupiter might witness, during the 
Jovian year, 4,500 eclipses of the moon (moons), and 
about the same number of the sun. 

Velocity of Light. — By an attentive examination 
of the eclipses of Jupiter's moons, Romer (a Danish 
astronomer), in 1617, discovered that the motion of 




llSillllill! 

light is not instantaneous, as was then believed. He 
noticed that the observed times of the eclipses were 
sometimes earlier and sometimes later than the 
calculated times, according as Jupiter was nearest 
or furthest from the earth. In Fig. 69, let J represent 
Jupiter ; e, one of the moons ; S, the sun ; and T and t, 
different positions of the earth in its orbit. When 



JUPITER. 163 

the earth is at T, the eclipse occurs 16 min. and 36 
sec. earlier than at t. That interval of time is 
required for the light to travel across the earth's 
orbit, giving a velocity of about 186,000 miles per 
second. 

Jupiter's Belts are dusky streaks of varying 
breadth and number, lying more or less parallel to 
the planet's equator. A brighter, often rose-colored, 
space marks the equatorial regions. The belts are 
not permanent, but change sometimes in the course 
of a few hours. Occasionally, only two or three 
broad belts are visible ; at other times, a dozen 
narrow ones appear. Often, spots are seen that are 
more lasting than the dark stripes. * It is now sup- 
posed that the planet is enveloped in dense clouds, 
through which light cannot penetrate, and that the 
globe itself is heated to a high degree, and gives off 
vapors, f The parallel appearance is doubtless due 
to strong equatorial currents, analogous to our trade- 
winds. 

* In 1878, a "Great Red Spot" appeared in the southern hemisphere of Jupiter. 
Its length was estimated at 8,000 miles, and its breadth at 2,000 miles. This curious 
phenomenon is still visible, but much diminished in brightness (1884). 

t Jupiter and Saturn are older planets than the earth and Mars, but, being so large, 
they have cooled more slowly, and are yet only partially solidified, so that Jupiter, at 
least, still shines with much of its primeval fire. Mars typifies the middle age ; Saturn 
and Jupiter, the youth ; and Uranus and Neptune, the infancy of planetary existence. 
In the case of Saturn and Jupiter, we never see the real planets, but only the outline of 
their atmospheres. If this theory be true, Jupiter and Saturn now represent the con- 
dition in which our earth existed ages ago, before a solid crust had been formed upon its 
surface.— (Geology, p. 17.) 



164 THE SOLAR SYSTEM. 

VIII. SATURN. 

The god of time. Sign ? , an ancient scythe. 

Description. — We now reach, in our outward jour- 
ney from the sun, the most remote world known to 
the ancients. It shines with a steady pale yellow 
light, which distinguishes it from the fixed stars. 
Its orbit is so vast that its movement among the 
constellations may be easily traced through one's 
lifetime. It requires two and a half years to pass 
1 through a single sign of the zodiac ;* hence, when 
once known, it may be readily found again. The 
earth leaves it at conjunction, makes a yearly rev- 
olution about the sun, comes to its starting point, 
and overtakes Saturn in about thirteen days there- 
after, f It is smaller than Jupiter, but more gor- 
geously attended. Besides a retinue of eight satel- 
lites, it is surrounded by a system of rings, some 
shining with a golden light, and others transparent, 
— a spectacle as wonderful as it is unique. 

Motion in Space. — Saturn revolves about the sun 
at a mean distance of nearly 886,000,000 miles. The 
eccentricity of its orbit is a trifle more than that of 
Jupiter, so that while it may, at perihelion, come 
fifty million miles nearer than its mean distance, at 
aphelion it swings off as much beyond. We can 
form some estimate of the size of its immense orbit, 
when we remember that it is moving 22,000 miles 

* Because of its slow, dreary pace, Saturn was chosen by the ancients as the symbol 
for lead. 

t From this the year of Saturn may be determined- As 13 : 378 days : : Earth's 
year : Saturn's year = 30 yr. nearly. 



SATURN. 



165 




per hour, and yet, from night to night, we can 

scarcely detect any F ig. 70, 

change of place. 

The Saturnian year 

is equal to about 

thirty of ours, and 

comprises nearly 

25,000 Saturnian 

days, each about 

10J- hours long. 

The Distance 
from the Earth is 
found in the same Saturn - 

manner as that of the other superior planets, being 
least in opposition and greatest in conjunction. Ac- 
cording as the earth and Saturn occupy different 
portions of their orbits, the distances between them 
at different times may vary nearly 300,000,000 miles. 

Dimensions. — The diameter of Saturn is about 
73,000 miles. Its volume is 700 times that of the 
earth. Its density is about f that of water, or a 
little more than that of pine wood. The Saturnian 
force of gravity is therefore scarcely greater than 
the terrestrial, so that a stone would fall toward 
the surface of that immense globe only about seven- 
teen feet the first second. 

Seasons, — The light and heat of the sun at Saturn 
are only T ^ that which we receive. The axis of 
Saturn is inclined from a perpendicular to the plane 
of its orbit about 31°.* The seasons therefore are 



* Proctor says 26° ; others, 28°. Let the pupil adapt the paragraph to each of these 
estimates. 



166 THE SOLAR SYSTEM. 

similar to those of the earth, but on a larger scale. 
The sun climbs in summer about 8° higher above the 
horizon, and sinks correspondingly lower in winter. 
The tropics are 16° further apart, and the arctic and 
antarctic circles 8° further from the poles. Each of 
Saturn's seasons lasts more than seven of our years. 
There is an interval of fifteen years between the 
autumn and spring equinoxes, and between the sum- 
mer and winter solstices. For fifteen years, the sun 
shines on the north pole, and a night of the same 
length envelopes the south pole. The atmosphere is 
doubtless very dense, as the belts seem to indicate. 

Telescopic Features. — Saturn's Rings. — Galileo 
first noticed something peculiar in the shape of 
Saturn. Through his imperfect telescope it seemed 
to have on each side a small planet, like a supporter, 
to help old Saturn on his way. Galileo therefore an- 
nounced to his friend Kepler the curious discovery, 
that " Saturn is threefold." As the planet, however, 
approached its equinoxes, these attendants vanished 
from his instrument. This was a great perplexity to 
the philosopher, and he never solved the mystery. 
When the rings were afterward seen, their real 
form was not known. They were supposed to be a 
kind of handle attached to the planet. 

Description of the Rings. — The series consists of 
three rings of unequal breadth, surrounding the 
planet at the equator. The exterior ring is separated 
from the middle one by a distinct break, while the 
interior ring seems joined to the middle one. They 
differ in their brightness ; the exterior ring is of a 
grayish tint ; the middle one is the most brilliant. 



SATURN. 167 

being more luminous than Saturn itself ; the interior 
one is darker and has a purple tinge. The two outer 
rings are known as the bright rings, and the inner 
one is called the dusky ring. The exterior and mid- 
dle rings are both opaque and cast on the planet a 
distinct shadow ; while the interior one is so trans- 
parent that it appears upon the globe of Saturn as a 
dark band through which the surface of the planet 
is readily seen. 

Saturn's Rings. (Proctor.) 

Miles. 

Diameter of exterior ring 166,920 

Breadth of exterior ring 10,000 

Diameter of middle ring 144,300 

Breadth of middle ring 17,600 

Distance between exterior and middle ring 1,700 

Diameter of interior ring 92,000 

Breadth of interior ring 8,600 

Distance of interior ring from the planet 10,000 

Entire breadth of ring system 37,570 

Thickness of rings, less than 100 

Rotation.— The rings revolve around Saturn in 
about 10J hours, in the same direction as the planet 
rotates on its axis. The globe of Saturn is not 
exactly at the center of the rings. This fact, com- 
bined with the rotary motion, is essential to the 
stability of the rings, preventing them from being 
precipitated upon the planet. 

Phases of the Rings. — The plane of the rings is 
inclined about 28° to the ecliptic. In its revolution 
about the sun, the axis of Saturn remaining parallel 
to itself, the sun sometimes illumines the northern 
and sometimes the southern face of the rings. At 
Saturn's equinoxes, only the edge receives the light. 



168 



THE SOLAR SYSTEM. 



and the rings are invisible to us, except with the 
most powerful telescopes, and then only as a line of 
light. The body of the planet constantly cuts off 
the sun's rays from a portion of the rings, and also 
serves to conceal from our view some of the lumin- 




Phases of Saturn's Rings. 



ous part. By a careful study of the cut, these various 
positions of the planet and rings, with the favorable 
times for observation, may be understood. 

Composition of the Rings. — It is now generally 
believed that the rings consist of a cloud of tiny 
satellites, — too small to be seen with the telescope,— 
revolving about the planet (see Nebular Hypoth- 
esis). 

Belts, — The surface of Saturn is traversed by faint 



SATURN. 



169 



dusky belts of a far less distinct and definite appear- 
ance than those upon Jupiter. The equatorial re- 
gions are more strongly marked than the other parts 
of the disk. 

Composition of the Planet. —It is quite probable 
that Saturn, like Jupiter, has no solid crust, but con- 
sists of molten matter surrounded by vapor that con- 
tinually rises from the heated interior (note, p. 163). 

Satellites. — Saturn has eight satellites. 



Names of Saturn's 
Satellites. 



I j Mimas 

II Enceladus.. 

Ill j Tethys 

IV i Dione 

V Rhea 

VI Titan 

VII Hyperion . . 

VIII I Japetus. . . . 



Distance from 


Approximate 


Sidereal 


Saturn in 


diameter in 


Period in 


miles. 


miles. 


days. 


120,800 


1,000 


0.94 


155,015 




1.37 


191,248 


500 


1.88 


245,876 


500 


2.73 


343,414 


1,200 


4.51 


796,157 


3,300 


15.94 


1,006,656 


9 


21.29 


2,313,835 


1,800 


79.33 



Titan is the largest, and in size exceeds Mer- 
cury. Enceladus and Mimas are the faintest of 
twinklers, and can be seen only with a powerful 
telescope. They were first detected by Herschel, 
''threading like pearls the silver line of light," to 
which the ring, then seen edgewise, was reduced, — 
advancing off it at either end, returning, and then 
hiding themselves behind the planet. The first 
three of these moons are nearer to Saturn than our 
moon is to the earth, but Japetus is nearly ten times 
as distant : so that the diameter of the Saturnian 
system is nearly four and a half million miles. 

Saturnian Scenery. — The magnificence of the 



170 



THE SOLAR SYSTEM. 



scenery upon Saturn must surpass anything with 
which we are familiar. In the cut, is given an ideal 
view of a landscape located upon the planet at a lati- 
tude of about 28°, taken at midnight. The rings form 
an immense arch, which spans the sky and sheds a 




Mi. 



Ideal Landscape, on Saturn, supposing a solid crust to exist. 




soft radiance around ; while, to add to the strange 
beauty of the night, eight moons in all their different 
phases — full, new, crescent, or gibbous — light up the 
starry vault. 



IX. URANUS. 

" Heaven," the most ancient of the gods. Sign, I£ ; H, the initial letter of Hersehel, 
with a planet suspended from the cross-bar. 

Description. — On the 13th of March, 1781, between 
10 and 11 p. m., Sir William Hersehel was examining 



URANUS. 171 

with his great telescope some stars in the constella- 
tion Gemini. A small star attracting his attention, 
he observed it with a higher magnifying power, 
when, unlike the fixed stars, its disk widened. 
Watching it for several nights, he detected its mo- 
tion in space ; but, mistaking its true character, he 
announced the discovery of a new comet. A few- 
months examination revealed the error, and the new 
body — new to us, but older perhaps than our own 
world* — was admitted to be a member of the solar 
system. 

Uranus may be seen in a dark sky, by a person of 
strong eyesight, if he previously knows its exact 
position among the stars. Its f aintness is due to its 
great distance from the earth. Were it as near as 
the sun, it would appear twice as large as Jupiter. 

Motion in Space. — Uranus revolves about the sun 
at a mean distance of nearly 1,782,000,000 miles. Its 
year exceeds eighty-four of ours. 

Dimensions. — Its diameter is about 33,000 miles. 
Its density is about equal to that of the water from 
the Dead Sea. The force of gravity upon the surface 
of the planet is T 9 o that upon the earth. 

Seasons. — We know little of the seasons of Uranus. 
If its axis lies in the plane of its orbit, the sun 
must wind in a spiral form around the planet. The 
light and heat are less than T ,^o of that which we 

* It is now known that Uranus had been previously observed by other astronomers. 
Le Monier at Paris had watched it for twelve successive nights, but pronounced it a 
fixed star. He had also seen it on previous occasions, and had he been an orderly ob- 
server, he would doubtless have detected its planetary character ; but he was extremely 
careless, as may be inferred from the fact related by Arago, that he had been shown one 
of Le Monier's observations of this planet written on a paper bag which originally con- 
tained hair-powder purchased at a perfumer's. 



172 THE SOLAR SYSTEM. 

receive; the light has been estimated to be about the 
quantity that would be afforded by three hundred 
full moons. The inhabitants of Uranus, if any such 
exist, can see Saturn, and perhaps Jupiter, but none 
of the planets within the orbit of the latter. 

Telescopic Features. — No spots or belts have been 
discovered. The time of rotation and the other 
features so familiar to us in the nearer planets are 
therefore unknown with regard to Uranus. 

Satellites. — Uranus has four moons, of which 
little is known except the curious fact that their 
orbits are nearly perpendicular to the plane of the 
planet's orbit, and that their movements are appar- 
ently retrograde — i. e., in the same direction as the 
hands of a watch. 



X. NEPTUNE. 

The god of the sea. Sign, f , his trident. 

Description. — Neptune is the far-off sentinel at the 
outpost of the solar system, being the most distant 
planet of which we have any knowledge. It is in- 
visible to the naked eye, and appears in the telescope 
as a star of the sixth magnitude. 

Discovery. — For many years, the motions of Ura- 
nus had been such as to baffle the most perfect calcula- 
tions. While far-distant Saturn, after his journey of 
thirty years, came around to his place true to the min- 
ute, Uranus defied arithmetic, and refused to con- 
form to the time set down for him on the heavenly 
dial. 



NEPTUNE. 173 

At length it was suggested that there was another 
planet exterior to Uranus, whose attraction produced 
these perturbations. So marked was this impression 
with Herschel, that he writes : "We see it as Colum- 
bus saw America from the shores of Spain. Its 
movements have been felt trembling along the far- 
reaching line of our analysis with a certainty not 
far inferior to ocular demonstration." 

Finally, two young mathematicians, Leverrier, of 
Paris, and Adams, of Cambridge, England^ each un- 
known to the other, set about the task of finding the 
place of this new planet. The problem was this : 
Given the disturbances produced by the attraction of 
the unknown planet, to find its orbit and its place in 
the orbit. 

Adams, after assiduous labor for nearly two years, 
completed his calculations and submitted them to 
Prof. Airy, the Astronomer Royal, in 1845. In the 
summer of 1846, Leverrier laid a paper before the 
Academy of Sciences in Paris, announcing the 
position of the unknown planet. Prof. Airy, hear- 
ing of this, was so impressed with the value of 
Adams's calculations, that he wrote to Prof. Challis, 
of Cambridge, to search that quarter of the heavens. 
Prof. Challis did as requested, and saw a star which 
afterward proved to be the planet so anxiously 
sought for. although at that time he failed to ascer- 
tain its true character. In September, of the same 
year, Leverrier wrote to Berlin, asking for assistance 
in searching for the planet. Dr. Galle, on receiv- 
ing the request, turned the large telescope of the 
Observatory to the place indicated, and almost im- 



174 THE SOLAR SYSTEM. 

mediately detected a bright star not laid down in the 
maps. This proved to be the predicted planet, found 
within less than a degree of the spot described by 
Leverrier. 

Such is the history of one of the grandest achieve- 
ments of the human mind. It stands as an ever 
fresh and assuring proof of the exactness of astro- 
nomical calculations, and the power of the intellect 
to understand the laws of the God of Nature. 

Motion in Space. — Neptune revolves about the sun 
at a mean distance of about 2,790,000,000 of miles. 
The Neptunian year is equal to nearly 165 terrestrial 
ones. Its motion in its orbit is the slowest of any of 
the planets, since it is the most remote from the sun. 
The velocity decreases from Mercury, which moves 
at the rate of about 105,000 miles per hour, to Nep- 
tune, whose rate is only 12,000 miles. 

Dimensions. — Neptune's diameter is about 37,000 
miles. Its volume is nearly 100 times that of the 
earth. Its density is a little less than that of Uranus. 

Seasons. — As the inclination of its axis is un- 
known, nothing can be ascertained concerning its 
seasons. The sun gives to Neptune but T ,ioo the 
light and heat which we receive. 

Though Neptune is at the extreme of the solar 
system, 2,790,000,000 miles beyond us, the same 
heavens bend above, the Milky Way is no nearer to 
the eye, and the fixed stars shine no more brightly. 
The planets, however, are all too near the sun to be 
seen, except Saturn and Uranus. The Neptunian 
astronomers, if there be any, are well situated for 
measuring the annual parallax of the stars, since 



METEORS AND SHOOTING STARS. 175 

Neptune has an orbit of 5,580,000,000 miles in 
diameter, and hence the angle must be thirty times 
as great as that which the terrestrial orbit affords. 

Telescopic Features. — On account of the recent 
discovery of this planet and its immense distance, 
nothing is known of its rotation or physical features. 

Satellites. — Neptune has one moon, at nearly the 
same distance from it as our own moon is from the 
earth. The revolution of this body about the planet, 
which is accomplished in about six days, has fur- 
nished the materials for calculating the mass of 
Neptune. 



III. METEORS AND SHOOTING 
STARS. 

Description. — All are familiar with those luminous 
bodies that flash through our atmosphere as if the 
stars were indeed falling from heaven. Different 
names have been applied to them, although the dis- 
tinction is not very definite. 

(1) Aerolites are those stony or iron masses 
which descend to the earth. 

(2) Meteors are luminous bodies which have a 
sensible diameter and a spherical form. They fre- 
quently pass over a great extent of country, and are 
seen for some seconds. Many leave behind them a 
train of glowing sparks ; others explode with reports 
like the discharge of artillery, — the pieces either 
continuing their course, or falling to the earth as 



17(5 



THE SOLAR SYSTEM. 



aerolites. Some meteors pass on into space ; some 
are vaporized ; while others are burned, and the ashes 
and fragments fall to the ground. 
(3) Shooting Stars are those evanescent, brilliant 




A Meteor with its Train. 



points that suddenly dart through the higher regions 
of the air, leaving a fiery train behind. 
1. Aerolites. — The fall of aerolites is frequently men- 



METEORS AND SHOOTING STARS. 177 

tioned and well authenticated. Chinese records tell 
of one as long ago as 616 B.C., that, in its fall, broke 
several chariots and killed ten men. A block of 
stone, equal to a full wagon-load, fell in the Helles- 
pont, B.C. 4:65. By the ancients, these stones were 
held in great repute. The Emperor Jehangire, it is 
related, had a sword forged from a mass of meteoric 
iron which fell in the Punjab in 1620. In 1795, amass 
was seen, by a ploughman, to descend not far from 
where he was standing. It threw up the soil on 
every side, and penetrated some distance into the 
solid rock beneath. In 1807, there was a shower of 
stones, one weighing 200 lbs., at Weston, Connect- 
icut. A mass once fell in South America, that was 
estimated to weigh fifteen tons. When first dis- 
covered, it was so hot as to prevent all approach. 
Upon its cooling, many efforts were made, by some 
travelers who were present, to detach specimens, 
but its hardness was too great for the tools that they 
possessed. In Yale College cabinet, there is a mass 
of meteoric iron, weighing 1,635 lbs. 

Aerolites consist of elements which are famil- 
iar. The analysis of these stellar objects gives us 
names as commonplace as if they had known a far 
less romantic origin, — iron, tin, copper, nickel, 
cobalt, lime, magnesia, oxygen, sulphur, phos- 
phorus ; in all, about twenty elements have been 
found. This fact is interesting as revealing some- 
thing of the chemistry of the region of space, concern- 
ing which we otherwise know little. The compounds, 
however, are so peculiar as to distinguish an aerolite 
from other substances. For example, meteoric iron. 



178 



THE SOLAR SYSTEM. 



a prominent constituent of aerolites, is an alloy that 
has never been found in terrestrial minerals. 

2. Meteors. — The records of meteors are even more 
wonderful than those of aerolites. It is related that 
at Crema, Italy, one day in the 15th century, the sky 
at noonday became dark, — a cloud of appalling black- 
ness overspreading the heavens. Upon this cloud, 
appeared the semblance of a great peacock of fire 




Copy of a Print Showing the Peculiar Crystalline Structure of Meteoric Iron. 

flying over the town. This suddenly changed to a 
huge pyramid, that rapidly traversed the sky. 
Thence arose awful lightnings and thunderings, 
amid which there fell upon the plain rocks, some 
of which weighed 100 lbs. In 1803, a brilliant fire- 
ball was seen traversing Normandy with great 
velocity, and some moments after, frightful explo- 
sions, like the noise of cannon, were heard coming 
from a black cloud hanging in the clear sky; they 



METEORS AND SHOOTING STARS. 179 

were prolonged for five or six minutes. These dis- 
charges were followed by a shower of heated stones, 
some weighing over 24 lbs. In 1819, a meteor was 
witnessed in Massachusetts and Maryland, the 
diameter of which was estimated at half a mile. In 
July, 1860, a brilliant fireball passed over the State 
of New York, from west to east, and was last seen 
far out at sea. On the evening of Feb. 12th, 1875, a 
magnificent meteor " illumined the entire State of 
Iowa, and parts of Missouri, Illinois, Wisconsin, and 
Minnesota. The aerolites that have been collected 
show its weight to have been fully 5,000 lbs." 

3. Shooting Stars. — One of the earliest accounts of 
star-showers is that which relates how, in 472, the 
sky at Constantinople appeared to be alive with fly- 
ing stars and meteors. In some Eastern annals we 
are told that in October, 1202, "the stars appeared 
like waves upon the sky. They flew about like 
grasshoppers, and were dispersed from left to right." 
It is recorded that in the time of King William II. 
there occurred in England a wonderful shower of 
stars, which "seemed to fall like rain from heaven. 
An eye-witness, seeing where an aerolite fell, cast 
water upon it, which was raised in steam, with a 
great noise of boiling." * 

Showers of 1799 and 1833.— The most remark- 
able accounts are those of the showers of November 
12th, 1799, and November 13th, 1833. Humboldt, in 
describing the former, says the sky was covered 



* Rastel says concerning it : "By the report of the common people in this kynge's 
time, diverse great wonders were seene, and therefore the kynge was told by diverse of 
his familiars that God was not content with his lyvyng." 



180 THE SOLAR SYSTEM. 

with innumerable fiery trails, which incessantly 
traversed the sky. From the beginning of the phenom- 
enon, there was not a space in the heavens three 
times the diameter of the moon that was not filled 
every instant with the celestial fireworks, — large 
meteors blending constantly their dazzling brilliancy 
with the long phosphorescent paths of the shooting- 
stars. (See notes, p. 305.) 

The latter shower was most brilliant on this conti- 
nent, and was visible from the lakes to the equator. 
Phosphoric lines swept over the sky like the flakes 
of a snow-storm. Large meteors darted across the 
heavens, leaving luminous trains behind them that 
were visible sometimes for half an hour : they gen- 
erally shed a soft white light ; occasionally, how- 
ever, yellow, green, and other colors varied the 
scene. Irregular fireballs, almost stationary, glared 
in the sky ; one especially, larger than the moon, 
hung in mid-air over Niagara Falls, and mingled its 
light with the foam and mist of the cataract. In 
many sections, the people were terror-stricken by 
the awful spectacle, and supposed that the end of 
the world had come. 

Inferior showers were seen in 1831, and 1832, and 
in the succeeding years, until 1839. These did not 
compare in brilliancy with the remarkable phenom- 
enon of 1833. There was an interval of about 34 
years between the great showers of 1799 and 1833 ; this 
seemed to indicate another shower in 1866 or 1867. 

In November, 1866, the people of both hemispheres 
were literally awake to the subject. Newspapers 
aroused the most sluggish imagination with thrilling 



METEORS AND SHOOTING STARS. 181 

accounts of the scones presented in 1799 and 1833. 
Extempore observatories were established at every 
convenient point. Watchmen were stationed, and 
the city bells were to be rung on the appearance of 
the first wandering celestial visitor. The exact 
night was not definitely known, but, for fear of a 
mistake, the 11th, 12th, and 13th were generally ob- 
served. The anxious vigils, the fruitless scannings 
of the sky, the disappointment, the meteors that 
were dimly thought to be seen, — all these were re- 
corded in the memory of the temporary astronomers 
of that year. 

While, however, the people of America were thus 
disappointed, there was enacted in England a dis- 
play brilliant indeed, though inferior to the one of 
1833. The staff at Greenwich Observatory counted 
about 8,000 meteors. 

In November, 1867, the long-expected shower was 
seen in this country, but it failed to satisfy the pub- 
lic anticipation. The sky was, however, illumined 
with shooting stars and meteors, some of which ex- 
ceeded Jupiter or Venus in brilliancy. 

Number of Meteors and Shooting Stars. — Prof. 
Newton estimates that the average number of me- 
teors that traverse the atmosphere daily, and which 
are large enough to be visible to the eye on a dark, 
clear night, is 7,500,000 ; and if to these the tele- 
scopic meteors be added, the number would be in- 
creased to 400,000,000. In the space traversed by 
the earth, there are, on the average, in each volume 
the size of our globe (including its atmosphere), as 
many as 13,000 small bodies, each one capable of 



182 THE SOLAR SYSTEM. 

furnishing a shooting star visible under favorable 
circumstances to the naked eye. 

Annual Periodicity of the Star- Showers. — On al- 
most any clear night, from five to seven shooting 
stars may be seen per hour, but in certain months 
they are much more abundant. Arago names the 
following principal dates : 

April 4-11 ; 17-25. October (about) 15. 

August 9-11. November 13-14. 

Origin. — Aerolites, meteors, and falling stars are 
produced by small bodies — planets in miniature — 
revolving, like our earth, about the sun. Their or- 
bits intersect the orbit of the earth, and if, at any 
time, they reach the point of crossing exactly with 
the earth, there is a collision. Their mass is so 
small, that the earth is not jarred any more than 
a railway train would be by a pebble thrown 
against it. 

These small bodies may come near the earth and 
be drawn to its surface by the power of attraction ; 
or they may sweep through the higher regions of 
the atmosphere, and then escape its grasp ; or, 
finally, they may, under certain conditions, be com- 
pelled to revolve many times around the earth as 
satellites. 

The November " meteoroids " (as these bodies are 
called before igniting) move at the rate of 26 miles 
per second in a direction nearly opposite that of the 
earth. They, therefore, meet our atmosphere with a 
relative velocity of 44 miles per second. As they 
sweep through the air, the friction partly arrests 



METEORS AND SHOOTING STARS. 183 

their motion, and converts it into heat and light. 
The body thus becomes visible to us. Its size and 
direction determine its appearance. If very small, 
it is consumed in the upper regions, and leaves only 
the luminous trail of a shooting star. If of large 
size, it may sweep along at a high elevation, or 
plunge directly toward the ground. Becoming 
highly heated in its course, it sheds a vivid light, 
while, unequally expanding, it explodes, throwing 
off large fragments which fall to the earth as 
aerolites, or continue their separate course as me- 
teors. The cinders of the consumed portion rain 
down on us as fine meteoric dust. * 

Meteoric Rings. — These little bodies, it is thought, 
do not generally revolve individually about the sun, 
but myriads of them are collected in a ring. 
When the earth passes through one of these floating 
girdles, a star-shower follows. This would account 
for their regular appearance at certain seasons of 
the year. The November meteoroids are not, like 
the August ones, uniformly distributed through the 
ring, but are principally collected in a swarm that 
has a period of 33£ years ; hence the August shower 
occurs quite regularly each summer, while the great 
November one happens only three times in a cen- 
tury. The orbit of the November stream extends 
beyond that of Uranus. The point where it crosses 
the earth's orbit moves forward about 50" per annum, 
and thus that star-shower occurs about a day later 
at each return. It takes three or four years for this 

* Prof. Young estimates that 100 tons of meteoric matter fall upon the earth daily 
from outer space. 



184 



THE SOLAR SYSTEM. 



swarm to pass the node, showing that the shoal of 
meteoroids occupies about ^ of its orbit. The earth 
in its annual revolution about 
the sun is supposed to en- 
counter several hundred of 
these meteoric rings. 

The Physical Relation be- 
tween meteoroids and comets 
is now generally acknowl- 
edged. The orbit of the Au- 
gust meteors is known to be 
identical with Comet III 1862 
(Swift's), and that of the 
November 14th shower cor- 
responds with Comet I 1866 
(Tempel's). The small show- 
ers of November 24 and 27 
are thought to be produced 
by meteors traveling in the 
path of the two dissevered 
parts of Biela's comet. 

The grand problem of me- 
teoric astronomy to-day is to 
identify the numerous mete- 
oric rings, and to detect their 
allied comets. Being thus 
intimately associated, they 
must have a common history. 
Prof. Newton, the great ad- 
vocate of this theory, broadly 
asserts that every meteoric 
stone was once a part of a comet, and every mete- 




Orbit of the August Meteors. 



COMETS. 185 

Oric shower consists of broken fragments of some 
known or unknown comet. 

Radiant Point. — The meteoroids are, of course, 
moving in parallel lines, but, by an optical illusion, 
they seem to radiate in all directions, the radiant 
point being in that part of the heavens which the 
earth is then approaching. * A star (v) in the blade 
of the sickle is the point from which the stars in the 
November shower radiate, while one in Perseus (/) 
is the radiant point of the August shower. 

Height. — Herschel estimates the average height of 
shooting stars above the earth to be seventy-three 
miles at their appearance, and fifty-two at their 
disappearance. 

Weight.— Prof. Harkness calculates that the aver- 
age weight of shooting stars does not differ much 
from one grain. 



IV, COMETS. 

We come now to notice a class of bodies the most 
fascinating, perhaps, of any in astronomy. The 
suddenness with which comets flame out in the 
sky, the enormous dimensions of their fiery trains, 
the swiftness of their flight, the strange and mys- 
terious forms they assume, their departure as un- 
heralded as their advent, — all seem to bid defiance 
to law, and partake of the marvellous. Superstitious 

* The same illusion is seen if, looking upward, we watch snow-flakes falling during a 
calm. Those coining directly toward our eyes seem to be motionless, and the rest to 
separate from them in diverging lines. This is the effect of perspective, and the "rad- 
iant point" is really the "vanishing point" of the parallel lines through which the 
meteors are moving. See Newcomb's Astronomy, p. 399. 



186 THE SOLAR SYSTEM. 

fears have been excited by their appearance, and 
they have been looked upon in every age as 

" Threatening the world with famine, plague, and war ; 
To princes, death ; to kingdoms, many curses ; 
To all estates, inevitable losses ; 
To herdsmen, rot ; to plowmen, hapless seasons ; 
To sailors, storms ; to cities, civil treasons." * 

Description. — The term comet signifies a hairy body. 
A comet consists usually of three parts ; — the nucleus, 
a bright point in the center of the head ; the coma 
(hair), the cloud-like mass surrounding the nucleus ; 

Fig. 76. Fig. 77. 





Comet without a Nucleus. Comet with a Nucleus. 

and the t ail, a luminous train extending generally in 
a direction opposite to the sun. There are comets 
without the tail, and others with several tails, while 
some are deprived of even the nucleus. The last 
consist merely of a fleecy mass, known to be a 
comet from its orbit and rapid motion. 

* Thus the comet of 43 b. c. , which appeared just after the assassination of Julius 
Caesar, was looked upon by the Romans as a celestial chariot sent to convey his soul 
heavenward. A.n old English writer observes: "Cometes signifie corruptions of the 
ayre. They are signes of earthquakes, of warres, of changyng kyngedomes, great 
dearthe of corn, yea, a common death of man and beast." Another remarks : "Experi- 
ence is an eminent evidence that a comet, like a sword, portendeth war ; and a hairy 
comet, or a comet with a beard, denoteth the death of kings, as if God and nature in- 
tended by comets to ring the knells of princes, esteeming bells in churches upon earth 
not sacred enough for such illustrious and eminent performances." 



COMETS. 187 

Comets are not confined, like the planets, to the 
limits of the zodiac, but appear in every quarter of 
the heavens, and move in every conceivable direc- 
tion. When first seen, the comet resembles a faint 
spot of light upon the dark background of the sky: 
as it approaches the sun the brightness increases, 
and the tail begins to show itself. Generally it is 
brightest near perihelion, and gradually fades away 
as it recedes, until it is finally lost, even to the 
telescope.* 

The Time of Greatest Brilliancy depends some- 
what on the position of the earth. If, as represented 
in the figure, the earth is at 

° ' Fig. 78. 

a when the comet, moving to- 
ward perihelion, is at r, the 
comet will appear more dis- 
tinct than when it is more dis- 
tant at P, although at the latter 
point it is really brighter. If, 
however, the earth is at c 
at the time of perihelion, the 

comet will be much more conspicuous. Again, if 
the earth is passing from a to b during the time 

* While a comet remains in regions beyond the planets, where the temperature is be- 
low — 140° C, its matter must be chiefly solid or liquid. On its approach to the sun, its 
enveloping atmosphere (if none existed, one will now be formed) will expand, and the 
nucleus will appear, surrounded by a blaze of light, feeble at first, but becoming more 
and more brilliant, and so producing the head, or coma, of the comet. Many comets do 
not go beyond this first phase, and, being exposed only to a moderate heat, remain 
telescopic. Others, piercing further the solar system, and reaching a higher tempera- 
ture, develop a more abundant atmosphere. The sun, while attracting to himself the 
nucleus, has power to repel some of the matter of the atmosphere ; how or why, we 
know not. Enough, that certain parts fly off as if driven by a gale, so making the tail, 
which increases more and more until the atmosphere is exhausted. Meanwhile, remark- 
able changes take place in the nucleus. Eruptions occur. Pieces are sometimes thrown 
off large enough to form a new comet, and showers of spark-like particles, with occasion- 
ally stony masses, fill the orbit of the comet with meteoroids.— Schiaparelli. 




188 THE SOLAR SYSTEM. 

the comet is near the sun, it will appear less brill- 
iant than if the earth were moving from c to d, as 
we should then be much nearer it during its greatest 
illumination. 

Number of Comets. — Kepler remarks that " there 
are as many comets in the heavens as fish in the 
sea." Arago, basing his calculations on the number 
known to exist between the sun and Mercury, has 
estimated that there are 17,500,000 within the solar 
system. Of this vast number, few are visible to the 
naked eye, and a still less number attract observa- 
tion, owing to their inferior size and brilliancy. 
Many are doubtless lost to our sight by being above 
the horizon in the daytime. During the eclipse of 
1882, Lockyer, who was in Egypt to take observa- 
tions, saw a brilliant comet near the sun. 

Orbits of the Comets. — Comets form a part of the 
solar system, and are subject to the laws of gravita- 
tion. Like the planets, they revolve around the sun, 
though they differ in the form of their orbits. While 
the planets move in paths varying but little from 
circular, and thus never depart so far from the sun as 
to be invisible to us, the comets travel in extremely 
elongated (flattened) ellipses, so that they can be ob- 
served by us through only a small portion of their 
paths. 

In Fig. 79 are represented the three general classes 
of cometary orbits. A comet traveling along an 
elliptical orbit, though it may pass far from the sun, 
will yet return within a fixed time ; one pursuing 
either a parabolic or hyperbolic curve cannot return, 
as the two sides separate from each other more and 






COMETS. 



189 



more. Many of the comets of the first class have been 
calculated, and they have repeatedly visited our por- 
tion of the heavens ; while those of the other classes, 
having once visited our system, go away forever, 

Fig. 79. 




Three Forms of Cometary Orbits. 

seeking perhaps in the far-off space another sun, 
which in turn they will abandon as they have our own. 
Calculation of a Comet's Return. — As we can ob- 
serve so small a proportion of the entire orbit, it is 
very difficult, indeed oftentimes impossible, to decide 



190 THE SOLAR SYSTEM. 

whether it is an hyperbola, an ellipse, or a parabola, 
A few are known to move in elliptical paths, and 
their orbits have been so accurately computed that it 
is possible to predict the time of their appearance. 
The other comets may never return, or at least not 

Fig. 80. 




Projections of a few Cometary Orbits on the Plane of the Ecliptic. 

for centuries hence. They may be paying our sun 
their first visit ; or, if they have swept through the 
solar system before, it may have been at so remote a 
time that no record is preserved, even if it were not 
before the creation of man.. IJnjigr these circum- 



COMETS. 191 

stances, it is difficult to determine the place of these 
apparently erratic wanderers ; yet, in spite of all 
these obstacles, some have been tracked into space 
far beyond the telescopic view. For example, the 
comet of 1844 is announced to pay a visit to the 
astronomers of the year of our Lord 101,844. The 
period of the comet of 1744, is fixed at 122,683 
years. 

Distance from the Sun. — Some comets at their peri- 
helion sweep near the sun. Thus the one of 1680 came 
where the temperature was estimated by Newton to 
be about 2,000 times that of red-hot iron.* The near- 
est approach known is that of the comet of 1843, 
whose perihelion distance was but about 30,000 miles 
from the surface of the sun ; in fact, it doubled 
around that body in two-hours time. (Guillemin.)t 
The greatest aphelion distance yet estimated is that 
of the comet of 1844, which is over 400,000,000,000 
miles. The velocity varies, of course, with the 
position in the orbit. The comet of 1680 moved in 
perihelion at the rate of over two hundred and 
seventy-seven miles per second ; while in aphelion 
its velocity is only about six miles per hour. 

Density of Comets. — The quantity of matter con- 
tained in a comet is exceedingly small. Even tele- 
scopic stars are visible through the densest part. The 
comet of 1770 became entangled among Jupiter's 

* The comet of 1680 excited such terror in Europe that a medal was struck, to quiet 
the fears of the people. The inscription read thus: "The star threatens evil things; 
trust only ! God will turn them to good." Newton calculated the orbit of this comet 
and proved that the comet moves around the sun in obedience to the law of gravity. 

t The comet of 1843 excited much interest in this country since one Miller had pre- 
dicted that the end of the world would come in that year; his followers imagined this 
comet presaged the destruction of all things. 



192 THE SOLAR SYSTEM. 

moons, and remained there four months without in- 
terfering with their movements ; indeed, so far from 
that, its own orbit was so much changed by their 
proximity, that, from a periodical return of 5| years, 
it has not been seen since. We have good reason to 
suppose that the earth, in 1861. passed through the 
tail of a comet, its presence being indicated only by 
a peculiar phosphorescent mist. So that even should 
our earth run full-tilt against a comet, the shock 
might be quite imperceptible.* Still, however 
lightly we may speak of the probability of such a 
collision, we must remember that there are comets 
of greater solidity. Donati's, for instance, is esti- 
mated by some to be about y-Jo the mass of the earth. 
The concussion of such a body, moving with the 
speed of a cannon-ball, would undoubtedly produce 
a very sensible effect. 

It is not determined whether comets shine by their 
own or by reflected light. If, however, their nuclei 
consist of white-hot matter, a passage through such 
a furnace would be anything but desirable or satis- 
factory. After all the calculations of Astronomy, 
our only safety lies in that Almighty Power which 
traces the path and guides the course alike of 
planets and comets : He, whose eye marks the fall 

* " However dangerous might be the shock of a comet, it might he so slight that it 
would only do damage to that part of the earth where it actually struck ; perhaps, 
even, we might cry quits, if, while one kingdom were devastated, the rest of the earth 
were to enjoy the rarities which a body coming from so far might bring to it. Perhaps 
we should be very surprised to find that the debris of these masses that we despised 
were formed of gold or diamonds ; but who would be the more astonished — we or the 
comet -dwellers who would be cast upon our earth ? What strange beings each would 
find the other?" Lett re sur la Comete, par M. De Maupertuis. 

Young says, " It seems, on the whole, more probable that a comet is only a cloud of 
dust and vapor- a smoke-wreath— than that there is at the center any solid kernel. A 
comet is a mere airy nothing." 



COMETS. 193 

of the sparrow, sees as well the flight of the worlds 
He has created. 

Variations in Form and Dimensions. — Comets ap- 
pear to be subject to constant variations. They are 
now thought generally to decrease in brilliancy at 
each successive revolution about the sun. The same 
comet may present itself sometimes with a tail, and 
sometimes without. When the comet first appears, 
there is commonly no tail visible, and the light is 
faint. As it approaches the sun, however, its bright- 
ness increases, the tail shoots out from the coma, 
and grows daily in length and splendor. Supernu- 
merary tails, shorter and less distinct than the prin- 
cipal one, dart out, but they generally soon disap- 
pear, as if from lack of material. The tail of the 
comet of 1843, just after the perihelion, increased in 
length 5,000,000 miles per day. As the tail thus ex- 
tended, the nucleus was correspondingly contracted, 
so that this comet actually " exhausted its head in 
the manufacture of its own tail." 

Remarkable Comets. — Among the many comets 
celebrated in history, we shall notice only some of 
those that have appeared in the present century. 
The great comet of 1811 was a magnificent spectacle. * 
The head was 112,000 miles in diameter ; the nucleus 
was 400 miles ; while the tail, of a beautiful fan- 
shape, stretched out 112,000,000 miles. "The aphelion 
distance of this comet is fourteen times that of Nep- 
tune, or 40,000,000,000 miles. It is announced to re- 
turn in thirty centuries ! " To what profound depths 

* This was considered by the Russians to presage Napoleon's Invasion. 



194 



THE SOLAR SYSTEM. 



of space, beyond the solar system, beyond the reach 
of the telescope, must such a journey extend ! 



Fig. 81. 




Coggia's Comet, 187U. 

The Comet of 1835 is known as Halley's comet. 
This is remarkable as being the first comet whose 
period of revolution was satisfactorily established. 
Dr. Halley, on examining the accounts of the great 
comets of 1531, 1607, and 1682, suspected that they 
were the reappearances of the same comet, whose 
period he fixed at about 75 years.* He finally ven- 

* The history of this comet, as it has been traced back by its period of seventy-five 
years, is quite eventful. It was seen in England in 1066, when it was looked upon with 
dread as the forerunner of the victory of William of Normandy. It was then equal to 
the full moon in size. In 1456, its tail reached from the horizon to the zenith. It was 



COMETS. 



195 



tured to predict the return of the comet at near the 
end of 1758 or beginning of 1759. Although Halley 
did not live to see his prophecy fulfilled, great in- 
terest was felt in the result. It was not destined, 
however, for a professional astronomer to be the 




Donati's Comet. 

first to detect the comet. A peasant near Dresden 
saw it on Christmas night, 1758. 

supposed to indicate the success of Mahomet II., who had already taken Constantinople, 
and then threatened the whole Christian world. Pope Calixtus III., therefore, ordered 
extra Ave Marias to be repeated by everybody, and also the church bells to be rung 
daily at noon (whence originated the custom now so universal). A prayer was added as 
follows : " Lord, save us from the devil, the Turk, and the comet." In 1223, it was con- 
sidered the precursor of the death of Philip Augustus of France. The first recorded ap- 
pearance of Halley's comet was b. c. 130, when it was supposed to herald the birth of 
Mithridates. 



196 THE SOLAR SYSTEM. 

The Comet of 1843 was so brilliant that it was 
visible in full daylight. It was so near the sun at 
perihelion as " almost to graze his surface." 

Encke's Comet has a period of only 3| years. A 
most interesting discovery has been made from ob- 
servations upon its motion. The comet returns each 
time to its perihelion about 2 J- hours earlier than the 
calculations indicate. Hence, Prof. Encke has been 
led to conjecture that space is filled with a thin, 
ethereal medium capable of diminishing the centri- 
fugal force, and thus contracting the orbit of a comet. 

Donati's Comet (1858) was the subject of universal 
wonder. When first discovered, in June, it was 
240,000,000 miles from the earth. In August, traces 
of a tail were noticed, which expanded in October to 
about 50,000,000 miles in length. This comet, though 
small, has never been exceeded in the brilliancy of 
the nucleus and the graceful curvature of the tail. 
It will return in about 2,000 years. 

The "Great Comet of 1882" had, soon after pass- 
ing its perihelion, a nucleus as bright as a star of the 
1st magnitude, and a tail 60,000,000 miles long. The 
aphelion of its orbit is six times further than Nep- 
tune from the sun, and the comet's period is esti- 
mated at between eight and nine centuries. 



V, ZODIACAL LIGHT. 

Description. — If we watch the western horizon in 
March or April, just after sunset, we shall sometimes 
see the short twilight of that season illuminated by 






ZODIACAL LIGHT. 



197 



a faint nebulous light, of a conical shape, flashing 
upward, often as high as the Pleiades. In Septem- 
ber and October, at early dawn, the same appearance 



Fig. 83. 




Zodiacal Light 

can be detected near the eastern horizon. The light 
can be seen in this latitude only on the most favor- 
able evenings, when the sky is clear and the moon 
absent. Even then, it will be frequently confounded 
with the Milky Way or auroral lights. At the base, 



198 THE SOLAR SYSTEM. 

it is of a reddish hue, where it is so bright as often 
to efface the smaller stars. In tropical regions, the 
zodiacal light is perpetual, and shines with a bril- 
liancy sufficient, says Humboldt, to cast a sensible 
glow on the opposite part of the heavens. 

Origin. — The commonly-received opinion is, that it 
is caused by a faint, cloud-like ring, perhaps a 
meteoric zone, that surrounds the sun, and becomes 
visible to us only when the sun himself is hidden 
below the horizon. Others maintain that, since it 
has been seen in tropical regions in the east and the 
west simultaneously, it can be explained only on the 
theory of a "nebulous ring that surrounds the earth 
within the orbit of the moon." 



PRACTICAL QUESTIONS. 

1. Would the earth rise and set to a Lunarian ? 

2. Could there be a transit of Neptune ? 

3. Why does Mars's inner-moon rise in the west ? 

4. In what part of the sky do you always look for the planets ? 

5. Show how it was impossible for the darkness that occurred at the 
time of the Crucifixion of Christ to have been caused by an eclipse of the 
sun. 

6. Is there any danger of a collision between the earth and a comet ? 

7. How are aerolites distinguished ? 

8. When do we see the old moon in the west after sunrise ? 

9. When do we see the moon high in the eastern sky in the afternoon 
before the sun sets ? 

10. When is a planet morning, and when evening, star ? 

11. Is the sun really hotter in summer than in winter ? 

12. Why is a planet invisible at conjunction ? 

13. Must an inferior planet always be in the same part of the sky as the 
sun ? A superior planet ? 



PRACTICAL QUESTIONS. 199 

14. Why, in summer, does the sun, at rising and at setting, shine on 
the north side of certain houses ? 

15. What effect does the volume of a planet have upon the force of 
gravity at its surface ? 

16. In what part of the heavens do we see the new moon ? The old 
moon ? The crescent moon ? 

17. What is the Golden Number in the almanac ? 

18. Why do we have more lunar than solar eclipses ? 

19. In what direction do the horns of the moon turn ? 

20. Is the " tidal- wave " an actual movement of the water ? 

21. Why does the sun " cross the line " in some years on March 21, and, 
in others, on March 22 ? 

22. Do we ever see the sun where it really is ? 

23. At Edinburgh, Scotland, there are times when the sun rises at 3£ 
o'clock a. m. and sets at 8-| o'clock p. m., and the twilight lasts the entire 
night. When and why is this ? 

24. Which is the longest day of the year ? 

25. Is the moon nearer to us when it is at the horizon, or at the zenith ? 
26.- How many solar eclipses would happen each year if the orbits of the 

sun and the moon were in the same plane ? 

27. Is there any heat in moonlight ? 

28. Can we see the moon during a total eclipse ? 

29. Which of the planets are repeating a portion of the earth's history ? 

30. How many times does the moon turn on its axis each year ? 

31. Can you explain the different signs used in the almanac ? 

32. Show how the moon is a prophecy of the earth's future. 

33. Does the sun really rise and set ? 

34. Are the bright portions of the moon mountains or plains ? 

35. Which of the heavenly bodies are self-luminous ? 

36. Why is not a solar eclipse visible on the whole earth ? 

37. What is meant by the " mean distance " of a planet ? 

38. What keeps the earth in motion around the sun ? 

39. Do we ever see the sun after it sets ? 

40. When does the earth move the most rapidly in its orbit ? 

41. Have we conclusive evidence that any planet is inhabited ? 

42. When is the twilight the longest? The shortest ? Why ? 

43. What is a moon ? 



200 PRACTICAL QUESTIONS. 

44. To a person in the south temperate zone, where would the sun be 
at noon ? 

45. Is it correct to say that the moon revolves about the earth, when we 
know that, according to the law of Physics, they must both revolve about 
their common center of gravity ?* 

46. During a transit of Venus, do we see the body of the planet itself on 
the face of the sun ? 

47. How many real motions has the sun ? How many apparent ones ? 

48. How many real motions has the earth ? 

49. Can an inferior planet have an elongation of 90° ? 

50. How do we know the intensity of the sun's light on the surface of 
any of the planets ? 

51. Why is the Tropic of Cancer placed where it is ? 

52. What planets would float in water ? 

53. How must the moons of Jupiter appear during their transit across 
the disk of that planet ? 

54. " The shadow of the satellite precedes the satellite itself when Ju- 
piter is passing from conjunction to opposition, but follows it between 
opposition and conjunction." Explain. 

55. What facts point to the conclusion that Mars may, perhaps, have 
passed his planetary prime ? 

56. Why may we conceive that Saturn and Jupiter are yet in their 
planetary youth ? 

57. Show how, if the Nebular Hypothesis (p. 256) be accepted, the 
fashioning of a planet must require an enormous length of time. 

58. Do we know the cause of gravitation ? 

* " Strictly speaking, the moon does not revolve around the earth, any more than 
the earth around the moon ; but, by the principle of action and reaction, the center of 
each body moves around the common center of gravity of the two bodies. The earth 
being eighty times as heavy as the moon, this center is situated within the former, 
about three-quarters of the way from its center to its surface."— Newcomb's Astronomy, 
p. 91. 



III. 
THE SIDEREAL SYSTEM. 



" He telleth the number of the stars ; He calleth them all by their 
names. " 

Psalm cxlvii. 4. 



202 



I. The Stars., 



Fixed Stars not Seen. 

Parallax and Distance. 

Motion. 

Stars are Suns. 

Our Sun a Star. 

Solar System in Motion. 

Number of Stars. 

Scintillation. 

Magnitude. 

Cause of Difference in Brightness. 

11. Names. 

12. The Constellations. 

13. Invention of Constellations. 

14. Signs and Constellations not agreeing. 

15. Permanence of Constellations. 

16. Value of stars. 



10. 



II. The Constel- 
lations 



Northern Cir- j 
cumpolar Con- ~) 

STELLATIONS,fbr 

Latitude of New 
York V 



Ancient Views. 
IS. Three Zones. 
( 1. How traced. 
[ 2. Ursa Major. 
Ursa Minor. 



^ a. Description. 
b. Principal Stars. 

}■ c. Mythological Hist. 

! d. Distance of Polaris. 
) e. Latitude. 



I. Equatorial 
Constellations. 



3. The Southern 
Constellations. 



III. Double Stars, Star Clusters, 
Colored Stars, etc 



a. Description. 

b. Principal Stars. 

c. Mythological Hist. 



IV. Celestial Chemistry . 



VI. Celestial Measurements . 



4. Draco. 

5. Cepheus. 

6. Cassiopeia. 

1. How traced. 

2. Perseus. 

3. Andromeda. 

4. Aries. 

5. Taurus. 

6. Auriga. 

7. Pisces. 

8. Cetus. 

9. Gemini. 

10. Orion. 

11. Canis. 

12. Leo. 

13. Cancer. 

14. Virgo. 

15. Hydra. 

16. Canes Venatici. 

17. Berenice's Hair, 

18. Bootes. 
10. Hercules. 

20. Corona. 

21. Serpentarius. 

22. Libra. 

23. Sagittarius. 

24. Capricornus. 

25. Cygnus. 
V 26. Lyra. 
1-6. Double Stars, Colored Stars, Variable 

Stars, Temporary Stars, Star Clus- 
ters, Nebulae. 

7. Magellanic Clouds. 

8. The Milky Way. 

9. The Nebular Hypothesis. 

1. Spectrum Analysis. 

2. Spectroscope. 

3. Revelations Concerning Sun. 

4. Concerning Stars. 

5. Concerning Nebula. 

6. Concerning Solar Flames. 
1. Sidereal. 

a. Solar. 

3. Mean Solar. 

4. Sun-dial, etc. 

1. To Find Distance of Planets from Sun. 

2. To Find Moon's Distance from Earth. 

3. To Find Sun's Distance from Earth. 

4. To Find Longitude of a Place, etc. 



THE SIDEREAL SYSTEM. 



I. THE STARS. 



IN our celestial journey we have reached Nep- 
tune, the sentinel outpost of the solar system. 
We are now nearly 2,800,000,000 miles from our 
sun. Yet we are apparently no nearer the fixed stars 
than when we started. They twinkle as serenely 
there in the far-off sky as to us here on the earth. 
The heavens by night, with the exception of a few 
changes in the planets, look familiar. Between them 
and us there is still a vast chasm which no imagi- 
nation can bridge ; a distance so immense that figures 
are meaningless, and we can only call it space, — so 
profound that to us it is limitless, though beyond we 
see other worlds twinkling, like distant lights over a 
waste of waters. 

We never see the Stars. — This assertion seems 
paradoxical, yet it is strictly true. So far are the 
stars removed from us, that we see only the light they 
send, but not the surface of the worlds themselves. 
They are merely glittering points of light. The most 
powerful telescope fails to produce a sensible disk. 
This constitutes a marked difference between a planet 
and a fixed star. 



204 THE SIDEREAL SYSTEM. 

The Annual Parallax of the Fixed Stars. — When 
speaking of this subject on page 121, we said that 
186,000,000 miles, or the diameter of the earth's orbit, 
is the unit for measuring the parallax of the fixed 
stars. Yet when the stars are viewed from even 
these extreme points, they manifest so slight a 
change of place, that to estimate it is one of the most 
delicate feats of astronomy. 

At the present time, it is considered that the star 
Alpha (a) Centauri in the southern heavens is the 
nearest to the earth. Its parallax is judged to be 
about 1". Its distance is more than 200,000 times that 
of the earth from the sun, or twenty trillions of miles. 
This is probably by no means its actual distance, 
but merely the limit within which it cannot be, but 
beyond which it must be. * 

These figures convey to our mind no idea of dis- 
tance. Our imagination fails to grasp the thought, 
or to picture the vast void across which we are gaz- 
ing. We remember that light moves at the rate of 
186,000 miles per second. A ray at that speed would, 
in one day, plunge out into the abyss beyond Nep- 
tune six times the distance of that planet from the 
sun. Yet it must sweep on at this prodigious speed, 
day and night, for over 3J years to span the gulf 

* David Gill, the Royal Astronomer at the Cape of Good Hope, has recently deter- 
mined the parallax of a. Centauri to be 0".75. This would make its distance 275,000 
astronomical units. 275,000 x 93,000,000 miles = over 255 trillion miles. Light 
would require about 4\ years to travel this enormous distance. Vega's parallax is 
placed at not far from 0".2, which indicates a distance of about 1,000,000 astronomical 
units. Hence, Vega shines upon us from the inconceivable distance of ninety-three 
trillion miles! The parallax of Sirius has been variously estimated at from r '.16 to 
// .38. Newcomb places this star at more than a million radii of the earth's orbit away 
from us, yet its light is four times as brilliant as that of any other star. The difficulty 
of measuring the stellar parallax may be judged from the fact that 1" measures the angle 
at which a globe three-tenths of an inch in diameter would be seen when a mile away. 



THE STABS. 205 

and reach a stopping point at the nearest fixed star. 
It has been estimated that the average time re- 
quired for the light of the smallest stars which are 
visible to the naked eye to reach the earth is about 
125 years. What, then, shall we say of those far- 
distant ones, whose faint light appears as a mere 
fleecy whiteness even in the most powerful tele- 
scopes? The conclusion is irresistible, that the light 
we receive set out on its sidereal journey far back in 
the past, perhaps before the creation of man! 

Motion of the Fixed Stars. — It will aid us still 
further in comprehending the immense distances of 
the stars, to learn that, though they seem to be fixed, 
they are moving much more swiftly than any of 
the planets. Thus, Arcturus flies through space at 
the astonishing rate of 200,000 miles per hour, or 
nearly twice that of Mercury, and more than three 
times that of the earth. Yet, through all our life- 
time, we shall never be able to detect any change in 
its position. " It requires three centuries for it to 
move over the starry vault a space equal to the 
moon's apparent diameter." 

The Stars are Suns.— The vast distance at which 
the stars are known to be, precludes the thought of 
their shining, like the planets or the moon, by reflect- 
ing back the light of our sun. They must be self- 
luminous, and are doubtless each the center of a 
system of planets and satellites. 

Our Sun a Star. — As we see only the suns of these 
distant systems, so their inhabitants see only the sun 
of our system, and that as a small star. 
Our System in Motion. — Like all the other stars, 



M 



THE SIDEREAL SYSTEM. 



our sun is in motion. It is sweeping onward, with 
its retinue of worlds, 150,000,000 miles per year, toward 
a point in the constellation Hercules. The Pleiades 
has been thought to be the center around which this 




A part oj the Uonsteuatiou uf tue 1 



great movement is taking place, but most astron- 
omers consider the idea as a mere speculation. 

The Number of the Fixed Stars. — When we look at 
the heavens on a clear night, the stars seem innumer- 



THE STAKS. 207 

able. To count them, one would think almost as in- 
terminable a task as to number the leaves on the 
trees. It is, therefore, somewhat startling to learn 
that the entire number visible to the most piercing 
eyesight does not exceed 6,000, while few can dis- 
cern more than 4,000.* The number, however, which 
may be seen with a telescope is marvellous. In Fig. 
84, is shown a portion of the heavens where the 
naked eye sees but six stars. Could we examine the 
same region of the sky with more powerful instru- 
ments, new constellations would doubtless be des- 
cried in the infinite depths of space. 

Scintillation. — The twinkling of the fixed stars is 
due to what is termed in Physics the "Interference 
of Light." The air, being unequally dense, warm, 
and moist in its various strata, transmits very irregu- 
larly the different colors of which white light is com- 
posed. Now one color prevails over the rest, and 
now another, so that the star appears to alter its hue 
incessantly. As the purity and density of the air 
vary, the twinkling of the stars also changes, and, 
therefore, it is always greatest near the horizon. \ 

Magnitude of the Stars. — As the telescope reveals 
no disk of even the nearest stars, we know nothing 
of their comparative size. The finest spider's thread, 
placed at the focus of the instrument, hides the star 
from the eye. When the moon passes in front of a 

* This illusion may be easily explained, when we remember how the impression of a 
bright light remains upon the retina, as in the whirling of a firebrand. 

t Humboldt says that at Cumana, in South America, where the air is remarkably 
pure and uniform in density, the stars cease to twinkle after they have risen 15° above 
the horizon. This gives to the celestial vault a peculiarly calm and soft appearance.— It 
should be noticed that interference occurs only when the light emanates from a point. 
A body that subtends a visual angle, i. e., has a sensible disk, like a planet, cannot twinkle. 



20$ 



THE SIDEREAL SYSTEM. 



star, the occultation is instantaneous, and not gradual, 
as in the case of the planets. Classification depends, 
therefore, merely upon their relative brightness. 
The most conspicuous are termed stars of the first 



Fig. So. 




magnitude; of these there are about twenty. The 
number of second-magnitude stars in the entire 
heavens is sixty-five ; of the third, about 200 ; of the 
fifth, 1,100 ; of the sixth, 3,200 ; of the seventh, 13,000 ; 
of the eighth, 40,000 ; and of the ninth, 142,000. Few 
persons can see smaller stars than those of the fifth 
or sixth magnitude. 

The Difference in the Brightness of the stars may 
result from a difference in their distance, size, or 
intrinsic brightness. Hence it follows that the faint- 
est stars may not be the most distant from the earth. 

Names of Stars.— Many of the brightest stars 
received proper names at an early date ; as Sirius, 
Arcturus. The chief stars of each constellation are 
distinguished by the letters of the Greek alphabet ; 



z i 

H r, 




Alpha 

Beta 

Gamma 

Delta 

Epsilon 

Zeta 

Eta 

Theta 



The Greek Alphabet. 
I i Iota 
K k. Kappa 



Kho 



A A 

M fJL 

N v 

a f 



Lambda 

Mu 

Nu 

Xi 

Omicron 

Pi 



T t Tau 

Y v Upsilon 

$ <f> Phi 

X x Chi 

* ^ Psi 

O at Omega 






THE STARS. 209 

the brightest one being usually called Alpha (a), the 
next Beta (8), etc., — the name of the constellation, in 
the genitive case, being put after each. Ex., a Arie- 
tis, (3 Lyree.* 

Star catalogues are issued, containing the stars 
arranged in the order of their Right Ascension, and 
numbered for convenience of reference. Argelan- 
der's Charts have 324,188 stars marked in the north- 
ern hemisphere. 

The Constellations. — From the earliest ages, the 
stars have been arranged in constellations, for the 
purpose of more readily distinguishing them. Some 
of these groups were named from their supposed re- 
semblance to certain figures, such as perching birds, 
pugnacious bulls, or contorted snakes, while others 
do honor to the memory of classic heroes. 

' ' Thus monstrous forms, o'er heaven's nocturnal arch, 
Seen by the sage, in pomp celestial march ; 
See Aries there his glittering bow unfold, 
And raging Taurus toss his horns of gold ; 
With bended bow the sullen Archer lowers, 
And there Aquarius comes with all his showers ; 
Lions and Centaurs, Gorgons, Hydras rise, 
And gods and heroes blaze along the skies." 

With a few exceptions, the likeness is purely fan- 
ciful. Not only are the figures uncouth, and the 
origin often frivolous, but the boundaries are not 
distinct. Stars occur under different names ; while 
one constellation encroaches upon another, f Though, 

* This means a of Aries, /3 of Lyra ; the genitive case in Latin being equivalent to the 
preposition of. 

t Chambers well remarks, " Aries should not have a horn in Pisces and a leg in Cetus, 
nor should 13 Argos pass through the Unicorn's flank into the Little Dog. 51 Camelopar- 
dali might with propriety be extracted from the eye of Auriga, and the ribs of Aquarius 
released from 4(5 Capricorn!. " 



210 THE SIDEREAL SYSTEM. 

however, the constellations are thus rude and im- 
perfect, there seems little hope of any change. Age 
gives them a dignity that insures their perpetua- 
tion. 

The Invention of the Constellations goes back into 
ages of which no record remains. By some it has 
been ascribed to the Greeks. When the signs of the 
zodiac were named, they doubtless coincided with 
the constellations. Aries (the ram) was so called 
because it rose with the sun in the spring-time, and 
the Chaldean shepherds named it from the flocks, 
their most valued possession. Then followed, in 
order, Taurus (the bull) and Gemini (the twins), 
called from the herds, which were esteemed next in 
value. At the summer solstice, the sun appears to 
stop, and, crab-like, to crawl backward ; hence the 
name Cancer (the crab). When the sun is in Leo, 
the brooks being dry, the lion leaves his lurking- 
place and becomes a terror to all. Virgo comes 
next, when the virgins glean in the summer harvest. 
At the autumnal equinox, the days and nights are 
equally balanced, and this is beautifully represented 
by Libra (the scales). The vegetation decays in the 
fall, causing sickness and death ; the Scorpion, which 
stings as it recedes, is suggestive of this Parthian 
warfare. Sagittarius (the archer) tells of the hunt- 
ing month. Capricornus (the goat, which delights 
in climbing lofty precipices) denotes how at the 
winter solstice the sun begins to climb the sky on 
his return north. Aquarius (the water-bearer) is a 
natural emblem of the rainy season. Pisces (the 
fishes) is the month for fishing. 



THE STAES. 



211 



Signs and Constellations do not Agree. — By the 

precession of the equinoxes, as we have before de- 
scribed on page 106, the signs have fallen back along 



Fig. 86. 




The Signs and Constellations, us they now Compare in the Heavens, the former having 
fallen back, and the latter apparently advanced, 30° each. 



the ecliptic about 30°, so that those stars which were, 
during the infancy of astronomy, in the sign Aries 
(np) are now in Taurus (&), and those which were in 
the sign Pisces (X) are now in Aries (t).* 

Permanence of the Constellations. — The general 
appearance of the constellations and the figures 

* If the teacher will put a pin at the center of Fig. 86, and then draw a sharp knife 
between the signs and the constellations, so as to detach the middle of the cut, and 
cause the inner part to revolve, the signs may be turned before any constellation, and 
thus this change be clearly apprehended. 



212 THE SIDEREAL SYSTEM. 

which the stars form are due to the position we 
occupy. Could we cross the gulf of space beyond 
Neptune, the stars now so familiar to us would look 
strangely enough in their new groupings. As one in 
riding through a forest sees the trees apparently 
increase in size and open up to view before him, 
while they decrease in size and close in behind him, 
forming clusters and groups which constantly change 
as he passes along, so, as our earth travels with the 
solar system on its immense sidereal journey, 
the stars will gradually grow larger and brighter in 
front, while those behind us will appear smaller and 
dimmer. 

Since, in addition to this, the stars themselves are 
in motion with varying velocity and in different 
directions, the constellations must change still more 
rapidly, so as ultimately to transform the appear- 
ance of the heavens. In time, the " Bands of Orion " 
will be loosened, and the " Seven Sisters " will glide 
apart. Such are the distances, however, that, al- 
though these movements have been going on con- 
stantly, no variation has occurred, since the crea- 
tion of man, that is perceptible, save to the watchful 
astronomer. Nothing in nature is so invariable as 
the stars. They are the standards of time. Myriads 
of years must elapse before new maps of the constel- 
lations will be required. 

Value of the Stars in Practical Life. — " The stars 
are the landmarks of the universe." They seem to 
be placed in the heavens by the Creator, not alone to 
elevate our thoughts and expand our conceptions of 
the infinite and eternal, but to afford us, amid the 



THE STARS. 213 

constant fluctuations of our own earth, something 
unchangeable and abiding. Every object about us 
is constantly shifting, but over all shine the " eternal 
stars," each with its place so accurately marked, 
that to the astronomer and the geographer no decep- 
tion is possible. To the mariner, the heavens be- 
come a dial-plate, the figures on its face set with 
glittering stars, along which the moon travels as a 
shining hand that marks off the hours with an accu- 
racy no watch can ever rival. Standing on the deck 
of his vessel, far out at sea, a single observation of 
the sun or the stars decides his location in the waste 
of waters as accurately as if he were at home, and 
had caught sight of some old landmark he had 
known from his boyhood. In all the intricacies of 
surveying, the stars furnish the only immutable 
guide. Our clocks vainly strive to keep time with 
the celestial host. Thus, by an evident plan of the 
Creator, even in the most common affairs of life, are 
we compelled to look for guidance from the shifting 
objects of earth up to the heavens above. 

Ancient Views. — Anaximenes (500 b. c.) thought 
that the stars were for ornaments, and were nailed 
like bright studs into the crystalline sphere. Anaxa- 
goras considered that they were stones whirled up 
from the earth by the rapid motion of the ether, and 
that its inflammable properties set them on fire and 
caused them to shine as stars. Some schools of the 
Grecian philosophers — the Stoics, Epicureans, etc. — 
believed that they were celestial fires kept alive 
by matter that constantly streamed up to them from 
the center of the heavens. The stars were at one 



214 THE SIDEREAL SYSTEM. 

time said to feed on air ; at another, to be the breath- 
ing holes of the universe. 

Three Zones of Stars. — If we recall what was said 
on page 90, concerning the paths of the stars and the 
appearance of the heavens at different seasons of the 
year, we shall see that the constellations are nat- 
urally divided into three zones. The first embraces 
those which are visible through the entire year ; the 
second, those whose paths can be seen only in part 
on any given night ; and the third, those whose paths 
just graze our southern horizon, or never pass above 
it. 



II. THE CONSTELLATIONS. 

I. The Northern Circumpolar Constellations are 

visible in our latitude every night. They may be 
easily traced by holding the book up toward the 
northern sky in such a way that Polaris and the Big 
Dipper on the map and in the heavens agree in posi- 
tion, and then locating the other constellations by 
comparison. 

As the stars revolve about Polaris, their places will 
vary with every successive night through the year. 
The cut represents them as they are seen at midnight 
of the winter solstice. At 6 P. M. of that day, the 
right-hand side of the map should be held downward, 
and the Big Dipper will be directly below the north 
star. At 6 a. m., the left-hand side should be at the 
bottom, and the Dipper will be above Polaris. From 



THE CIRCUMPOLAR CONSTELLATIONS. 



215 



day to day, this aspect will change, each star coming 
a little earlier to the meridian, or to its position on 
the preceding night. The rate of this progression is 
six hours, or 90°, in three months. 



(Map No. 1.) Fig. 87. 




Northern Circumsolar Constellations. 

Ursa Major is represented under the figure of a 
great bear. It contains 133 stars visible to the naked 
eye. This constellation has been celebrated among 
all nations. It is remarkable that the shepherds of 
Chaldeain Asia and the Iroquois Indians of America 
gave to it the same name. 



216 THE SIDEREAL SYSTEM. 

Principal Stars. — A noticeable cluster of seven 
stars — six of the second and one of the fourth mag- 
nitude — forms what is familiarly termed the Dipper. 
In England it is styled Charles's Wain, from a 
fancied resemblance to a wagon drawn by three 
horses tandem. Mizar (£) has a minute companion, 
Alcor, which Humboldt tells us could be rarely seen 
in Europe. A person with good eyesight may now 
readily detect it. Megrez {$), at the junction of the 
handle and the bowl, is to be marked particularly, 
since it lies almost exactly in the colure passing 
through the autumnal equinox. Dubhe and Merak 
are termed the Pointers, because they point out the 
polar star. The bear's right fore-paw and hind-paw* 
are each marked by two small stars, as shown in the 
cut : a similar pair nearly in line with these denote 
the left hind-paw (see £, Fig. 00). 

Mythological History. — Diana had a beautiful attendant named 
Callisto. Juno, the queen of heaven, becoming jealous of the maid, trans- 
formed her into a bear. 

•• The prostrate wretch lifts up her head in prayer, 
Her arms grow shaggy, and deformed with hair ; 
Her nails are sharpened into pointed claws, 
Her hands bear half her weight and turn to paws. 
Her lips, that once would tempt a god, begin 
To grow distorted in an ugly grin. 
And lest the supplicating brute might reach 
The ears of Jove, she was deprived of speech. 
How did she fear to lodge in woods alone, 
And haunt the fields and meadows once her own ! 
How often would the deep-mouthed dogs pursue, 
"Whilst from her hounds the frighted hunters flew." 

* It is well to notice that Dubhe and Merak are about 5° apart ; Dubhe and Benet- 
nasch are about 25° apart ; the paws of the Beai are 15° apart ; while Polaris is about 
30° distant. 



THE CIRCUMPOLAR CONSTELLATIONS. 217 

Some time afterward, CalKsto's son, Areas, being out limiting, pursued 
his mother, and was about to transfix her with his uplifted spear, when 
Jupiter in pity transferred them both to the heavens, and placed them 
among the constellations as Ursa Major and Ursa Minor. 

Ursa Minor is represented under the figure of a 
small bear. It contains twenty-seven stars, of which 
only three are of the third, and four of the fourth 
magnitude. 

Principal Stars. — A cluster of seven stars forms 
the Little Dipper. Three of them are small, and are 
seen with difficulty. Polaris, at the extremity of the 
handle, has been known from time immemorial as 
the North Polar Star. Until the mariner's compass 
came into use, it was the star 

" Whose faithful beams conduct the wandering ship 
Through the wide desert of the pathless deep. " 

Polaris does not mark the exact position of the pole, 
since that is about 1^° toward the Pointers. This 
distance will gradually diminish,* until in time 
(2120 A. D.) it will be only J° : then it will increase 
again, until, in the lapse of ages, 12,000 years hence, 
the brilliant star Vega (a Lyrse) will fulfill the office of 
polar star for those who shall then live on the earth, f 
The Distance of Polaris is so great, that, though 
the star is moving through space at the rate of ninety 

* Five stars of the Dipper itself are drifting away from the sun, at the rate of 17 
miles per second, seeming to form a family or group by themselves. Proctor's Easy 
Star Lessons gives charts representing the appearance of the Dipper for 100,000 years. 

•f Of the nine Pyramids which are standing at Gizeh, Egypt, six have openings facing 
the north. These lead to straight passages which descend at a uniform angle of about 
26° and are parallel with the meridian. If we suppose a person, 4,000 years ago, stand- 
ing at the lower end of one of these passages, and looking out, his eye would strike the 
sky near the star Thuban, which was then the polar star. The supposed date of the 
building of these Pyramids (the Great Pyramid, 2123 b. c.) agrees with that epoch, 
and naturally suggests that the builders had some special design in this peculiar con- 
struction. 



218 THE SIDEREAL SYSTEM. 

miles per minute, this tremendous speed is impercep- 
tible to us. It requires nearly fifty years for its light 
to reach the earth : so that, when we look at Polaris, 
we know that the ray which strikes our eye set out 
on its journey through space half a century ago. 
We cannot state positively that the star is now in 
existence, since if it were destroyed to-day it would 
be fifty years before we should miss it. * 

Calculation of Latitude from Polaris. — By an 
observer at the equator, Polaris is seen at the horizon. 
If he goes north, the horizon is depressed, and Polaris 
seems to rise in the heavens. When it has reached 
the height of a degree, the observer is said to have 
passed over a degree of latitude on the earth's sur- 
face. As he moves further north, the polar star con- 
tinues to ascend ; its distance above the horizon 
denoting the latitude of each place in succession, 
until at the north pole, if one could reach that point, 
Polaris would be seen directly overhead. 

Draco is represented under the figure of a long 
sinuous serpent, stretching between Ursa Major and 
Ursa Minor, nearly encircling the latter constellation, 
and finally reaching out its head almost to the body 
of Hercules. 

Principal Stars. — Four small stars form a quad- 
rilateral figure at the head ; a fifth, of the fourth 
magnitude, which is scarcely visible, marks the end 
of the nose ; several scattered groups and little 
triangles of small stars denote the position of the 
various coils of the body ; thence, an irregular line 
of stars traces the dragon's tail around between Ursa 

* Some recent observations seem to reduce this to 42 years. 



THE CIRCUMPOLAR CONSTELLATIONS. 219 

Major and Ursa Minor. Thuban, lying midway be- 
tween y of the Little Dipper and £ of the Big Dipper, 
is noted as the polar star of forty centuries ago. 

Mythological History. — Jupiter had carried off Europa. Agenor, 
her father, sent her brother Cadmus in pursuit of his lost sister, bidding 
him not to return until he was successful in his search. After a time, 
Cadmus, weary of his wanderings, inquired of the oracle of Apollo concern- 
ing the fate of Europa. He was told to cease looking for his sister, to fol- 
low a cow as a guide, and when she rested, there to build a city. Hardly 
had Cadmus stepped out of the temple, when he saw a cow slowly walking 
along. He followed her until she came upon the broad plains where 
Thebes afterward stood. Here she stopped. Cadmus, wishing to offer a 
sacrifice to Jupiter in gratitude for the delightful location, sent his servants 
to seek for water. In a dense grove near by, was a fountain guarded by a 
fierce dragon (Draco), and sacred to Mars. The Tyrians, approaching this 
and attempting to dip up some water, were attacked, and many of them 
killed, by the enormous serpent, whose head overtopped the tallest trees. 
Cadmus, becoming impatient, went in search of his men, and, on arriving 
at the spring, saw the sad disaster. He forthwith fell upon the mon- 
ster, and after a severe battle succeeded in slaying him. While standing 
over his conquered foe, he heard a voice from the ground bidding him take 
the dragon's teeth and sow them. He obeyed. Scarcely had he finished 
when the earth began to move and the points of spears to prick through 
the surface. Next, nodding plumes shook off the clods, and the heads of 
armed men protruded. Soon a great harvest of warriors covered the entire 
plain. Cadmus, in terror at the appearance of those giants, whom he 
termed Sparti (the sown), prepared to attack them, when suddenly they 
turned upon themselves, and never ceased their warfare till only five of 
the crowd survived. These, making peace with one another, joined Cadmus, 
and assisted him in building the City of Thebes. 

Cephens is represented as a king in regal state, 
with a crown of stars on his head, while he holds in 
his hand a scepter which is extended toward his wife, 
Cassiopeia. The constellation contains thirty-five 
stars visible to the naked eye. 



220 THE SIDEREAL SYSTEM. 

Principal Stabs.— The brightest star is Alderamin 
(a), in the right shoulder. Alphirk ((I), in the girdle, is 
at the common vertex of several triangles, which point 
out respectively the left shoulder (t) } the left knee (y), 
and the right foot. The head, which lies in the Milky 
Way, is marked \)v a little triangle of three stars. 
This forms, with a, 3, and ', quite a regular quad- 
rilateral figure. A hright star of the fifth magnitude. 
close to Polaris, points out the left foot. 

Cassiopeia* is represented as a queen seated on 
her throne. On \k-j- right, is the king: on her left, 
Perseus, her son-in-law; above her. Andromeda, 
\i<-r daughter. The constellation contains sixty-seven 
stars visible to the naked eye. 

Principal Stars. — A line drawn from Megrez ( fl >), 
in Ursa Major, through Polaris and continued an 
equal distance-, will strike Caph (&) in Cassiopeia. 
This star is noticeable as marking, with the others 
named, tie- equinoctial colore, and as being on the 
same side of the true pole as Polaris. The principal 
stars form the figure of an inverter! chair, which is 
very striking and may be easily traced. 

II. Equatorial Constellations. — The constellations 
we shall now describe lie south of the circumpolar 
groups. Only a portion of their paths is above our 
horizon. In using the maps, the observer is supposed 
to stand with his back toward Polaris, and to be look- 
ing directly south. Commencing with the constella- 
tion Perseus, so intimately connected with the other 

the mythological hi-,*. ..a, see Perseus and Andromeda. The 

of the principal stars in the Chair make a mnemonic word,- fi'iyv., bogie. The student 
can often form raeb an association of the letters, and will find the device an aid to his 
memory. (Compare Virgo, page 230,) 



EQUATORIAL CONSTELLATIONS. 221 

members of the royal family just described, we pass 
eastward in our survey, and notice the various con- 
stellations as they slowly defile in silent march across 
the sky. 

The first map represents the constellations on or 
near the meridian at nine o'clock in the evening of 
the winter solstice. On the evening of the autumnal 
equinox, the left-hand side of the map should be 
turned downward toward the eastern horizon. On 
the evening of the vernal equinox, the right-hand 
side should be turned to the western horizon. At 
these different times, the stars, though keeping 
their relative positions, will be diversely inclined to 
the horizon. As the stars apparently move westward 
at the rate of 15° per hour, the time of the evening 
determines what stars will be visible, and also their 
distances above the horizon. 

Perseus is represented as brandishing an enor- 
mous sword in his right hand, while in his left he 
holds the head of Medusa. The constellation com- 
prises eighty-one stars visible to the naked eye. 

Principal Stars. — The most prominent figure is 
called the Segment of Perseus. It consists of several 
stars arranged in a line curving toward Ursa Major. 
Algenib (a), the brightest of these, is of the second 
magnitude. Algol (p. 242), in the midst of a group of 
small stars, marks the head of Medusa. Between the 
bright stars of Perseus and Cassiopeia, is a beautiful 
star-cluster visible to the naked eye. 

Mythological History. — Perseus, from whom this constellation was 
named, was the son of Jupiter and Danae. His grandfather, Acrisius, hav- 
ing been informed by the oracle that his grandson would be the instru- 



222 



THE SIDEREAL SYSTEM. 
(Map No. 2)- Fig. 88. 




ment of his death, put tho mother and child in a coffer and set them adrift 
on the sea. Fortunately, they floated near the island Seriphus, where they 
were rescued and kindly treated by a brother of Polydectes, king of the 
country. When Perseus had grown up, he was ordered by Polydectes 
to bring him, as a marriage gift, the head of Medusa. Now Medusa was 
oifee a beautiful maiden, who dared to compare her ringlets with those of 
Minerva ; whereupon, the goddess changed her locks into hissing serpents, 
and made her features so hideous, that she turned to stone every living 
object upon which she fixed her Gorgon gaze. Perseus was at first over- 
powered at the thought of undertaking this enterprise ; but Mercury 
promised to be his guide, and to furnish him with his winged shoes; Mi- 
nerva loaned him her wonderful shield, that was bright as a mirror ; and 
the Nymphs gave him, in addition, Pluto's helmet, which made the bearer 
invisible. Thus equipped, Perseus mounted into the air and flew to the 
ocean, where he found the three Gorgons, of whom Medusa was one, asleep. 
Fearing to gaze in her face, he looked upon the image reflected in Minerva's 
shield, and with his sword severed Medusa's head from her body. The 
blood gushed forth, and with it the winged steed Pegasus. Grasping the 
head, Perseus flew away. The other Gorgons awaking, pursued him, but 
he escaped their search by means of Pluto's helmet. As he flew over the wilds 



EQUATORIAL CONSTELLATIONS. 223 

of Libya, in his aerial route, the blood dripping from the gory head of the 
monster produced the innumerable serpents for which that country was 
afterward noted. 

Andromeda is represented as a beautiful maiden 
chained to a rock. 

Principal Stars. — Algenib and Algol in Perseus 
form, with Almach (y) in the left foot of Andromeda, 
a right-angled triangle opening toward Cassiopeia. 
This figure is so perfect, that the stars may be easily 
recognized. The girdle is pointed out by Merach (13), 
and two other stars which form a line slightly curv- 
ing toward the right foot. The breast is denoted by 
a very small triangle composed of three stars, — 6 of 
the fourth magnitude, another of the fifth magnitude 
just south, and an exceedingly minute star a little at 
the west. Alpheratz (a), in the head of Andromeda, 
belongs also to Pegasus. This star, with three others 
—Algenib (y), Markab (a), and Scheat (j3),— all of 
the second magnitude, constitute the Great Square 
of Pegasus. The brightest stars of these two con- 
stellations form a figure strikingly like the Big Dip- 
per. Algenib and Alpheratz lie in the equinoctial 
colure which passes through Caph. 

Mythological History. -Cassiopeia had boasted that her daughter 
Andromeda was fairer than the Sea-nymphs. They appealed, in great 
indignation, to Neptune, who sent a sea-monster (Cetus) to devastate the 
coast of Ethiopia. To appease the deities, her father Cepheus was directed 
by the oracle to bind his daughter to a rock, to be devoured by Cetus. 
Perseus, returning from the destruction of Medusa, saw Andromeda in her 
forlorn condition. Struck by her beauty and tears, he offered to liberate 
her at the price of her hand. Her parents joyfully consented, and, in 
addition, offered a royal dower. Perseus slew the terrible monster, and, 



224 THE SIDEREAL SYSTEM. 

freeing Andromeda, restored her to hsr parents. All the prominent actors 
in this scene were honored with seats among the constellations. The Sea- 
nyinphs, it is said, in petty spite of Cassiopeia, prevailed that she should 
be placed where for half of the time she hangs with her head downward, — a 
fit lesson of humility. Cepheus, her husband, shares in her punishment. 

Aries, the ram, was anciently the first constella- 
tion of the zodiac. It is now the first sign, but the 
second constellation. On account of the precession 
of the equinoxes, the constellation Pisces occupies 
the first sign. 

Principal Stars. — The most noted star is a Arietis 
(Alpha of Aries, more commonly called simply Arie- 
tis), in the right horn. This lies near the path of the 
moon and is one of the stars from which longitude is 
reckoned. A line drawn from Almach to Arietis 
will pass through a beautiful figure of three stars 
called The Triangles. 

Mythological History. — Phrixus and Helle were the children of 
Athamas, king of Thessaly. Being persecuted by Ino, their step-mother, 
they were compelled to flee for safety. Mercury provided them a ram 
which bore a golden fleece. The children were no sooner placed on his back 
than he vaulted into the heavens. In their aerial journey, Helle becoming 
dizzy fell off into the sea, which was afterward called the Hellespont, now 
the Dardanelles. Phrixus having reached Colchis in safety, offered the ram in 
sacrifice to Jupiter, and gave the golden fleece to Aetes, his protector. The 
Argonautic expedition in pursuit of this golden fleece, by Jason and his 
followers, is one of the most romantic of mythological stories. It is, 
undoubtedly, a fanciful account of the first important maritime expedition. 
Rich spoils were the prizes to be secured. 

Taurus consists of the head and shoulders of a 
bull, which is represented in the act of plunging at 
Orion. 

Principal Stars. — The Hyades, a beautiful cluster 



EQUATORIAL CONSTELLATIONS. 225 

in the head, forms a distinct V. The brightest of 
these is Aldebaran, a fiery red star of the first mag- 
nitude.* The Pleiades (Job, xxxviii, 31), or the Seven 
Sisters, is the most conspicuous group in the sky 
(p. 206). It contains a large number of stars, six of 
which are visible to the naked eye. There were said 
to have been seven anciently, but that Electra left 
her place in order not to behold the ruin of Troy, 
which was founded by her son Dardanus. Other 
myths relate that the "Lost Pleiad" was Merope, 
who married a mortal. Alcyone is the brightest 
Pleiad. El Nath {$) and £ point out the horns of 
Taurus. 

Mythological History. — This is the animal whose form Jupiter 
assumed when he bore off Europa. The Pleiades were the daughters of 
Atlas, and Nymphs of Diana's train. They were distinguished for their 
unblemished virtue and mutual affection. The hunter Orion having pur- 
sued them one day, in their distress they prayed to the gods, when Jupiter, 
in pity, transferred them to the heavens. 

Auriga, the Charioteer or Wagoner, is represented 
as a man resting one foot on a horn of Taurus, and 
holding a goat and kids in his left hand and a bridle 
in his right. 

The Principal Stars are arranged in an irregular 
five-sided figure. Capella, the goat-star, is of the 
first magnitude. It travels in its orbit 1,800 miles per 
minute ; seventy years— a long lifetime —are required 
for its light to reach the earth. Near by is a tiny 
triangle, formed of three small stars, called the Kids. 
Menkalinan (#) is in the right shoulder, in the right 

* Aklebaran is estimated to move through the heavens at the rate of 55 miles per 
second. (See pp. 205, 261.) A number of the bright stars between Aldebaran and the 
Pleiades have a common motion of about 10" per century toward the east. 



226 



THE SIDEREAL SYSTEM. 



hand, (3 (common to Auriga and Taurus) the right 
foot, and i the left foot. Capella, (3, and d (a star in 
the head) form a triangle. The origin of this con- 
stellation is unknown. 

Pisces, the fishes, is represented by two fishes tied 
together by a long ribbon. It consists of small stars, 
which can be traced only upon a clear night, and in 
the absence of the moon. 

Cettis, the whale, is a huge sea-monster, slowly 
ploughing his way eastward, midway between the 
horizon and the zenith. It may easily be found, on 
a clear night, by means of the numerous figures 
given in the map. 

(Map No. 3)— Fig. 89. 




Gemini, the Twins, represents the twin brothers 
Castor and Pollux. 

* " Castor is resolved by the telescope into two stars, whose angular distance from 
each other is 5"— the angle that one inch would subtend 1,146 yards off."— Ball. 



EQUATORIAL CONSTELLATIONS. 227 

The Principal Stars are Castor* and Pollux, which 
are of the first and second magnitudes. The latter 
is one of the stars from which longitude is reckoned 
by means of the Nautical Almanac. The constella- 
tion is clearly distinguished by two nearly parallel 
rows of stars, that by a slight effort of the imagina- 
tion may be extended into the constellations Taurus 
and Orion. 

Mythological History. — Castor and Pollux were noted, — the former 
for his skill in training horses, the latter for boxing. They were tenderly 
attached to each other, and were inseparable in their adventures. They ac- 
companied Jason on the Argonautic expedition. A storm having arisen 
during this voyage, Orpheus played on his wonderful lyre and prayed to the 
gods ; whereupon th« tempest was stilled, and star-like flames shone upon 
the heads of the twin-brothers. Sailors, therefore, considered them as 
patron deities,* and the balls of electric flame seen on masts and shrouds, 
now called St. Elmo's fire, were named after them. Afterward, Castor was 
slain. Pollux being inconsolable, Jupiter offered either to take him up to 
Olympus, or to let him share his immortality with his brother. Pollux pre- 
ferred the latter, and so the brothers pass alternately one day under the 
earth, and the next in the Elysian Fields. Not only did sailors thus con- 
fide in their watch over navigation, but soldiers believed them to return, 
mounted on snow-white steeds and clad in rare armor, to take part in the 
hard-fought battle-fields of the Romans. 

" Back comes the chief in triumph, 
Who in the hour of fight 
Hath seen the great Twin Brethren, 

In harness on his right. 
Safe comes the ship to haven, 

Through hillows and through gales, 
If once the great Twin Brethren 
Sit shining on the sails."— Lays of Ancient Rome. 

Orion is represented under the figure of a hunter 
assaulting Taurus. He has a sword in his belt, a 

* We rememher that Paul sailed for Italy in a ship whose sign was Castor and Pollux. 
--Acts, xxviii. 11. 



228 THE SIDEREAL SYSTEM. 

club in his right hand, and the skin of a lion in his 
left. This is one of the most clearly defined and con- 
spicuous constellations in the heavens. 

Principal Stars. — Four brilliant stars, in the form 
of a parallelogram, mark the outlines of Orion. Betel- 
geuse, a beautiful ruddy star of the first magnitude, 
is in the right shoulder ; Bellatrix (y), of the second 
magnitude, is in the left shoulder ; Rigel, of the first 
magnitude, is in the left foot ; and Saiph (*), of the 
third magnitude, is in the right knee. Two small 
stars near A form with it a small triangle, which is 
itself the vertex of a larger triangle composed of A, 
Y, and Betelgeuse. Xear the center of the parallel- 
ogram are three stars forming the Belt of Orion. 
This group is also called the Bands of Orion (Job, 
xxxviii, 31), Jacob's rod, and the Yard. It received 
the last name because it forms a line 3° long, divided 
in equal parts by a star in the center. These divi- 
sions are useful for measuring the distances of the 
stars. Running from the belt southward, is an 
irregular line of stars which marks the sword ; west 
of Bellatrix is a curved line denoting the lion's skin. 
South of Orion are four stars forming a beautiful 
figure styled The Hare. 

Mythological History. — Orion was a famous hunter. Becoming 
enamored of Merope, he desired to marry her. (Enopion, her father, 
opposing the choice, put out the eyes of the unwelcome suitor. The 
blinded hero followed the sound of a Cyclop' s hammer until he came to 
Vulcan's forge. Vulcan, taking pity, instructed Kedalion to conduct him 
to the abode of the sun. Placing his guide on his shoulder, Orion pro- 
ceeded to the east, and at a favorable place 

" Climbing up a narrow gorge, 
Fixed his blank eyes upon the sun " 



EQUATORIAL CONSTELLATIONS. 229 

The healing beams restored him to sight. As a punishment for having 
profanely boasted that he was able to conquer any animal the earth could 
produce, he was bitten in the heel by a scorpion. Afterward, Diana placed 
him among the stars ; where Sirius and Procyon, his dogs, follow him, the 
Pleiades fly before him, and far remote is the Scorpion, by whose bite he 
perished. 

Canis Major and Canis Minor contain each a 
single star of the first magnitude, Sirius, and Pro- 
cyon.* These two, with Betelgeuse, Phaet in the 
Dove, and Naos in the Ship, form a huge figure 
known as the Egyptian X, Sirius, the dog-star, is 
the most brilliant star in the heavens. It is reced- 
ing from the earth at the rate of 20 miles per second 
(Huggins). Seventeen years are required for its light 
to reach us.f (See note, p. 308.) 

Leo is represented as a rampant lion. It is one of 
the most beautiful constellations in the zodiac. 

The Principal Stars are arranged in the form of 
a sickle. Regulus, in the handle, is a brilliant star of 
the first magnitude. It is one of the stars from 
which longitude is reckoned. It is almost exactly in 
the ecliptic. Zosma (8) lies in the back of the lion, 
6 in the thigh, and Denebola, a star of the second 
magnitude, in the brush of the tail. 

Cancer includes the stars that lie irregularly 
scattered between Gemini, Head of Hydra, Procyon, 
and Leo. In the midst of these, is a luminous spot, 
called Prsesepe, or the Bee-hive, which an ordinary 
glass will resolve into stars. 

* Procyon, like Sirius, was formerly considered a star of evil omen, and as bringing 
bad weather. " Who that is learned in matters astronomical," said Digges, the astrol- 
oger, "noteth not the great effects at the rising of the star called the Litel Dogge." 

t In 1862, Alvan G. Clark, son of the famous telescope-maker, discovered a companion 
of Sirius, " distant from the star 28 times the Sun's distance from the Earth," 



230 THE SIDEREAL SYSTEM. 

Virgo is represented as a beautiful maiden with 
folded wings, bearing in her left hand an ear of corn. 

The Principal Star, Spica, in the ear of corn, is of 
the first magnitude, and is used for determining lon- 
gitude at sea. Denebola, Cor Caroli (a), Arcturus, 
and Spica form a figure about 50° in length, called 

(Map No. U)- Fig. 90. 




the Diamond of Virgo. Five third-magnitude stars, 
£ ? $, 7> V, P> (the mnemonic word is begde) make a 
corner known among the Arabian astronomers as 
"The retreat of the howling dog." 

Mythological History. — Virgo was the Goddess Astraea. According 
to the poets, the early history of man was the golden age. It was a time 
of innocence and truth. The gods dwelt among men, and perpetual spring 
delighted the earth. Next, came the silver age, less tranquil and serene. 



EQUATORIAL CONSTELLATIONS. 231 

but still the gods lingered and happiness prevailed. Then followed the 
brazen and iron ages, when wickedness reigned supreme. The earth was 
wet with slaughter. The gods left the abodes of men, one by one, Astrsea 
alone remaining ; until finally she too, last of all the immortals, bade 
the earth farewell. Jupiter thereupon placed her among the constellations. 

Hifdra is a long, straggling serpent, having its 
head near Procyon and extending its tail beyond 
Virgo, a total distance of more than 100°. 

The Principal Star is Cor Hydree, of the second 
magnitude. 'It is a lone star, and may be easily 
found by a line drawn from y Leonis through Regu- 
lus, and continued about 23°. The head is marked 
by a rhomboidal figure of four stars of the fourth 
magnitude lying near Procyon. Several little 
triangles may be formed of them and other small 
stars lying near. The Crater, or Cup, is a beautiful 
and very striking semicircle of six stars of the fourth 
magnitude directly south of 6 Leonis. Corvus (§, e, 
y, 6), the raven, lies 15° east of the Cup. e Corvi is in 
the equinoctial colure. 

Mythological History. — Hydra was a fearful serpent which in ancient 
times infested the lake Lerna. Its destruction constituted one of the 
twelve labors of Hercules. The Crow was formerly white, it is said, but 
was changed to its raven tint on account of its proneness to tale-bearing. 

Canes Venatici, the hunting dogs. This constel- 
lation contains the bright star, Cor Caroli (a), which is 
found by a line passing from Benetnasch (?/) through 
Berenice's Hair to Denebola (13). 

Berenice's Hair is a beautiful cluster midway 
between Cor Caroli and Denebola. Near by is a 
single bright star of the fourth magnitude, 



232 THE SIDEREAL SYSTEM. 

Mythological History. — Berenice was the wife of Ptolemy. Her 
husband going upon a dangerous expedition, she promised to consecrate 
her beautiful tresses to Venus if he should return in safety. Soon after the 
fulfilment of this vow the hair disappeared from the temple where it had 
been deposited. Berenice being much disquieted at this loss, Conon, the 
astronomer, announced that the locks had been transferred to the heavens, 
in proof of which he pointed out this cluster of hitherto unnamed stars. 
All parties were satisfied with this happy termination of the difficulty. 

Bootes, the bear-driver, is represented as a hunts- 
man grasping a club in his right hand, while in his 

(Map No. 5)— Fig. 91. 




left he holds by the leash his two greyhounds (Canes 
Venatici), with which he is pursuing the Great Bear 
continually around the north pole. 

Principal Stars. — Arcturus (Job, ix, 9), a mag- 
nificent star of the first magnitude, is in the left 
knee. It forms a triangle with Denebola and Spica, 
and also one with Denebola and Cor Caroli. It 
travels in its orbit fifty-five miles per second, or 



EQUATORIAL CONSTELLATIONS. 233 

three times as fast as the earth (p. 205). Its light 
reaches the earth in twenty-five years. Mirach (e) lies 
in the girdle, 6 in the right shoulder, Alkaturops (/x) 
in the club, (3 in the head, and Seginus (y) in the left 
shoulder. Seginus forms with Cor Caroli and Arc- 
turus a triangle, right-angled at Seginus. Three 
small stars in the left hand of Bootes lie near 
Benetnasch. 

Mythological History.— Bootes is supposed to have been Areas, the 
son of Callisto. (See Ursa Major.) 

Hercules is represented as a warrior clad in the 
skin of the Nemeean lion, holding a club in his right 
hand and the dog Cerberus in his left. His foot is 
near the head of Draco, while his head lies 38° south 
and his club reaches 10 degrees beyond. 

The Principal Star is Has Algethi {a Herculis). 
This forms a triangle with (3 and 6. A peculiar 
figure of four stars (n } rj, £, e), north of these, marks 
the body. (See Maps, Nos. 5, 6, and 7.) The left 
knee is pointed out by 6, and the left foot by y. 

Mythological History. — This constellation immortalizes the name of 
one of the greatest heroes of antiquity. Hercules was the son of Jupiter 
and Alcmena. While he was yet lying in his cradle, Juno, in her jealousy, 
sent two serpents to destroy him. The precocious infant, however, 
strangled them with his hands. By the cunning artifice of Juno, Hercules 
was made subject to Eurystheus, his elder half-brother, and compelled to 
perform all his commands. Eurystheus enjoined upon him a series of the 
most difficult and dangerous enterprises that could be conceived, which have 
been termed the " Twelve Labors of Hercules. " Having completed these 
tasks, he afterward achieved others equally celebrated. Near the close of 
his life he killed the centaur Nessus. The dying monster charged Dejanira, 
the wife of Hercules, to preserve a portion of his blood as a charm to use 



234 THE SIDEREAL SYSTEM. 

in case the love of her husband should ever fail her. In time, Dejanira 
thought she needed the potion, and Hercules having sent for a white robe 
to wear at a sacrifice, she steeped the garment in the blood of Nessus. No 
sooner had Hercules put on the fatal robe than the venom stung his bones 
and boiled through his veins. He attempted to tear it off, but in vain. 
It stuck to his flesh, and tore off great pieces of his body. The hero, 
finding he must die, ascended Mount (Eta, where he erected a funeral pyre, 
spread out the skin of the Nemaean lion, and laid himself down upon it. 
Philoctetes applied the torch. With perfect serenity of countenance 
Hercules awaited approaching death — 

" Till the god, the earthly part forsaken, 

From the .man in flames asunder taken, 

Drank the heavenly ether's purer breath. 

Joyous in the new unwonted lightness 

Soared he upward to celestial brightness, 

Earth's dark, heavy burden lost in death."— Schiller. 

Corona consists of six stars arranged in a semi- 
circular form. The brightest of these is Alphecca. 
This makes a triangle, with Mirach (e) and 6 in Bootes. 
It forms a similar figure with Mirach and Arcturus. 

Serpcntarius, or Ophin chits, the serpent-bearer, 
is represented under the figure of a man grasping in 
both hands a prodigious serpent, which is writhing 
in his grasp. 

Principal Stars. — Ras Alhague (a), in the head, is 
of the second magnitude. It is about 5° from Ras 
Algethi. They form a pair of stars conspicuous like 
the pairs in Gemini, Canis Minor, Canis Major, etc.; 
(5 marks the right shoulder, and k the left. There is 
a small cluster near ft called Taurus Poniatowskii. 
An irregular square of four stars, near y Herculis, 
denotes the head of the serpent. 

Mythological History. — This constellation perpetuates the memory 
of JEsculapius, the father of medicine. He was so skilful that he restored 
several persons to life ; whereupon Pluto complained to Jupiter that his 



EQUATORIAL CONSTELLATIONS. 



235 



kingdom was in danger of being depopulated. Therefore Jupiter struck 
him with a thunderbolt, but afterward placed him among the constel- 
lations. Serpents were sacred to iEsculapius, because of the superstitious idea 
that they have the power of renewing their youth by changing their skin. 

Libra represents the scales of Astrsea (Virgo), the 
goddess of justice. It may be recognized by the 

(Map No. 6)— Fig. 92. 




quadrilateral figure formed by its four principal 
stars. 

Scorpio is represented under the figure of a huge 
scorpion, stretching through 25°. It is a most in- 
teresting constellation. 

Principal Stars. — Antares (a) is a fiery red star of 
the first magnitude. It marks the heart of the Scor- 
pion. The head is indicated by several stars, the 



236 THE SIDEREAL SYSTEM. 

most prominent of which is /?, arranged in a line 
slightly curved. The tail may be easily traced by a 
series of stars which winds around through the Milky 
Way in a beautiful manner. * 

Mythological History. — This is the scorpion that sprang out of the 
earth at the command of Juno, and stung Orion. Scorpio and Orion are 
so placed among the constellations that they never appear in the heavens 
together. 

Sagittarius, the archer, is represented as a cen- 
taur with his bow bent, as if about to let fly an arrow 
at Scorpio. 

Principal Stars. — A row of stars from \i to /? marks 
the bow : another from y eastward points out the 
arrow and the right arm drawn back in bending the 
bow. North of r, two stars of the fourth magnitude 
denote the head of the centaur. The Milk Dipper, 
so called because the handle lies in the Milky Way, 
is a very striking figure. 

Mythological History. — This constellation is named in honor of 
Chiron, one of the centaurs. These monsters were represented as men from 
the head to the loins, while the remainder of the body was that of a horse 
— the ancients having so high an opinion of that animal that the union 
was not considered in the least degrading. 

Chiron was renowned for his skill in music, medicine, and prophecy. 
The most distinguished heroes of mythology were among his pupils. He 
taught iEsculapius physic ; Apollo, music ; and Hercules, astronomy. 

Capricomus contains no very conspicuous stars. 
The Southern Fish (No. 6) has one star of the first 
magnitude, Fomalhaut (a, No. 7), which on a clear 
summer evening may be seen in the south, midway 
to the zenith. Antinous and the Eagle is a double 

* Antares (anti, like ; Ares, Mars) was so named because it rivalled Mars in brightness 
and color. 



EQUATORIAL CONSTELLATIONS. 237 

constellation. It contains a beautiful star of the 
first magnitude, Altair. This is conspicuous, as be- 
ing the center one in the row of three bright stars. 
A similar row denotes the tail of the eagle ; the first 
star of which is named £, and the last star lies in 
Cerberus. The Dolphin contains a pretty cluster in 
the form of a diamond. It is sometimes called Job's 
Coffin. 

(Map No. 7)— Fig. 93. 




Cygmis, the swan, is a remarkable group of stars, 
the principal ones being so arranged as to form a 
large and beautiful cross. The upright piece lies 
along the Milky Way. It is composed of four stars, 
three of which, Deneb (a), y, and 8, are bright, while 
the fourth is a variable star. No. 61, a minute star, 
scarcely visible to the naked eye, is noted as being 
the nearest to the earth of any of the fixed stars in 
the northern hemisphere (p. 241). 



238 



THE SIDEREAL SYSTEM. 



Lyra, the harp, contains one brilliant blue star*, 
Vega (p. 217). Close by it is a parallelogram of four 
smaller stars, by which it may be easily recognized. 

Mythological History. — This is the celestial lyre upon which Orph- 
eus discoursed such ravishing music that wild beasts forgot their fierceness 
and gathered about him to listen, while the rivers ceased to flow, and the 
very rocks and trees stood entranced. 

III. Southern Constellations. — We now imagine 
ourselves viewing the stars visible to a person far 

(Map No. 8)— Fig. 94. 




south of the equator. The constellations are re- 
versed with reference to the horizon. The two stars 
which, in the northern hemisphere, compose the base 
of the parallelogram in Orion, form here the upper 
side. Sirius is above Orion. All the northern cir- 
cumpolar constellations are hidden from view. At 
the southern pole there is no conspicuous star, but 
the richness and number of the neighboring stars 
compensate this deficiency, and give to the heavens 



THE SOUTHERN CONSTELLATIONS. 239 

an incomparable splendor. Here is the magnificent 
constellation Argo, in which we find Canopus, looked 
upon anciently as next to Sirius in brilliancy : 77, a 
variable star, now surpasses it in brightness. 

Nearly at the height of the south pole, blazes the 
Southern Cross; below is the Centaur, containing 
two stars of the first magnitude and five of the 
second ; and above is Hydrus, where shines Acher- 
nar, another beautiful star of the first magnitude. 

(Map No. 9)— Fig. 95. 




III. DOUBLE STARS, COLORED 
STARS, NEBULAE, ETC. 

1. Double Stars. — To the naked eye, all the stars 
appear single. With the telescope, over 10,000 have 
been found to be double. Thus, Polaris consists of 
two stars about 18" apart; Bigel has a companion 
about 10" from it ; and Sirius, one distant 7". A 



240 THE SIDEREAL SYSTEM. 

good opera-glass will separate e Lyrse into two 
components. 

In case two stars happen to lie in the same straight 
line from us, though at immense distances from each 
other, their light will blend. They will be seen by 
the naked eye as a single star, and by the telescope 
as a double star. They are called optical double 
stars. Many, however, of the double stars have been 
found to be physically connected. Each double star 
of this class forms a binary system of two suns re- 
volving in an elliptical orbit about their common 
center of gravity, like the planets in the solar system, 
in accordance with Newton's law of gravitation. In 
a few instances, there are combinations of triple, 
quadruple, and even septuple stars. Thus e Lyrse is 
a double-double star, while 6 Orionis is a system of 
six suns. The components of a double star com- 
monly differ in brightness ; so that frequently the 
fainter one is nearly lost in the brilliancy of its com- 
panion sun. 

The Periods of some systems have been ascer- 
tained. Thus, £ Ursse Ma j oris is a double star, and 
the two stars of which it is composed have performed 
an entire revolution about each other since they 
were found to be connected. There are only eleven 
binary stars now known whose periods are less 
than a century, while the others have periods which 
seem to extend, in some cases, beyond a thousand 
years. 

Orbits. — It is not possible to estimate the dimen- 
sions of the orbits of the double stars, until their dis- 
tances from us are definitely known. " Taking the 



DOUBLE STARS. 241 

estimated distance of 61 Cygni (550,000 times the 
sun's distance from the earth)* as a basis, the com- 
panions of that system cannot cultivate a very 
intimate acquaintance, since they must be over a 
billion miles apart. From these data, astronomers 
have attempted even to calculate the mass of some 
of the double stars. 61 Cygni, although scarcely 
visible to the naked eye, and known to be the sec- 
ond nearest to us of any of the fixed stars, is esti- 
mated to weigh one-third as much as our sun." (See 
p. 308.) 

II. Colored Stars. — We have already noticed that 
the stars are of various colors, f Sirius is white; 
Antares, red; and Capella, yellow ; while Lyra has a 
blue tint, and Castor has a green one. In the pure 
transparent atmosphere of tropical regions, the 
colors are far more brilliant. There, oftentimes, 
the nocturnal sky is a blaze of jewels, — the stars 
glittering with the green of the emerald, the blue 
of the amethyst, and the red of the topaz. 

In the double and multiple stars, every color is 
presented in all its richness and beauty ; while there 
are also combinations of colors complementary to 
each other. Here is a green star with a blood-red 
companion ; here an orange and a blue sun ; there a 
yellow and a purple one. The triple star y Andro- 
medae is formed of an orange-red sun and two others 
of an emerald green. 

Every tint that blooms in the flowers of summer, 

* Recent measurements of this star seem to indicate its probable distance from the 
sun to be 400,000 radii of the earth's orbit. 

t The theory has been advanced that the color indicates the intensity of the heat of 
the star. A white star is therefore hotter than a red star ; and a blue, than a yellow one. 



242 THE SIDEREAL SYSTEM. 

flames out in the stars at night. "The rainbow 
flowers of the footstool and the starry flowers of the 
throne," proclaim their common Author ; while rain- 
bow, flower, and star alike evince the same Divine 
love of the beautiful. 

We can hardly conceive the effects produced in a 
system having colored suns. Take a planet revolv- 
ing about \p Cassiopeise for instance. This is illu- 
minated by a red, a blue, and a green sun. Some- 
times, by the succession of these suns, a cheerful 
green day would present a charming relief to a fiery 
red one ; and that might be still further subdued by 
a gentle blue one. The odd contrast of color and the 
vicissitudes of extreme heat and cold that obtain on 
such a world, present a picture which our fancy can 
sketch better than words can paint. 

The colors of the stars change. Sirius was ancient- 
ly red. It is now unmistakably white. There are 
two double stars which were described by Herschel 
as white ; each is now composed of a golden-yellow 
and a greenish star. 

III. The Variable Stars have periodic changes of 
brilliancy. The following are most conspicuous : 

Algol, in the head of Medusa, is a star of the 
second magnitude for about two and a half days, 
when it suddenly decreases, and in three-and-a-half 
hours descends to the fourth magnitude. It then re- 
kindles, and in three-and-a-half hours is again as 
brilliant as ever. 

Mira, the wonderful, a star in the Whale, has a 
period of eleven months. It is ordinarily of the 
second magnitude for about fifteen days. It then 



DOUBLE STARS. 243 

decreases for three months, until it becomes invisible 
to the naked eye. This period of darkness lasts five 
months ; it then rebrightens for three months, until 
it regains its former lustre. Occasionally, however, 
it fails to brighten at all beyond the fourth magni- 
tude, while on one occasion its light was almost equal 
to that of Aldebaran. Sometimes no perceptible 
change takes place for a month ; then again, there is 
a sensible alteration in a few days. 

The Reason of this Variability is not under- 
stood. It has been suggested, in the case of Mira, 
that it may be a globe rotating on its axis, and that 
different portions of its surface, illuminated to differ- 
ent degrees of intensity, are thus presented to us. 
Others have conceived that there may be satellites 
revolving about these suns, and that when their dark 
bodies interpose between the stars and our earth, 
they eclipse the light wholly or in part. 

IV. The Temporary Stars suddenly blaze out in 
the heavens, and then gradually fade away. The 
most celebrated one burst forth in Cassiopeia, in the 
year 1572. Tycho Brahe says : " One night as I was 
examining the celestial vault, I saw with unspeak- 
able astonishment a star of extraordinary brightness 
in Cassiopeia. Struck with surprise, I could scarcely 
believe my eyes. To convince myself that there was 
no illusion, I called the workmen of my laboratory 
and the passers-by, and asked them if they saw the 
star which had so suddenly made its appearance. 
It could be compared only with Venus at her quad- 
rature, being seen distinctly at midday." Its color 
was at first white, then yellow, and finally red, Its 



244 THE SIDEREAL SYSTEM. 

brightness decreased gradually until the spring of 
1574, when the star disappeared from view and has 
not since been seen. As two brilliant stars had pre- 
viously appeared in Cassiopeia, at intervals of about 
three centuries, they have been thought, by some, to 
be identical, and that it is only a variable star of long 
period. 

Since this discovery by Tycho Brahe, numerous in- 
stances are recorded of stars which have suddenly 
burst forth, and have then either faded out entirely, 
or remain as faint telescopic objects. In the latter 
case, they are termed New stars. One of this kind 
appeared in Corona Borealis, in 1866. At first it was 
of the second magnitude, but in a week changed to 
the fourth, and in a month diminished to the ninth. 
Strangely, too, some stars have disappeared from the 
heavens, and are styled Lost stars. The changes 
which are thus constantly taking place are calculated 
to make the term "eternal stars" seem a very in- 
definite phrase. 

Explanation. — These phenomena are as yet little 
understood. A rotation about an axis would fail 
to explain the changes in color. Some think that 
these stars revolve in enormous orbits of such eccen- 
tricity that at their most distant points they fade out 
of sight. Arago has shown, in reply to this, that for 
a star to decrease in brightness from the first magni- 
tude to the second by moving directly from us, even 
with the velocity of light, would require six years. 
As we have just seen, the star of 1866 underwent this 
change in brilliancy in a week. 

The mind cannot help wondering if they are not 



DOUBLE STARS. 245 

instances of enormous conflagrations in which a 
world is overwhelmed in ruin ! The investigations 
of spectrum analysis indicate that the star of 1866 
consisted of burning hydrogen gas. We can suppose 
that the gas was evolved by some convulsion, and, 
taking fire, wrapped the entire globe in flames. 
This need not involve the idea of destruction, but 
only a change of form. A dark star may thus 
become luminous, or a bright one may be extin- 
guished. * 

5. The Star Clusters are groups of stars so massed 
together as to present a hazy, cloud-like appearance. 
Several of them have been already named, — the Plei- 
ades, the Beehive in Cancer, Berenice's Hair, the 
Hyades, and the group in the sword-handle of Per- 
seus. The principal stars of which they are composed 
can generally be distinguished by the naked eye, 
although by the use of a small opera or spy-glass the 
number is increased. 

In the southern sky, there are clusters still more 
remarkable. In the Cross, is a group of 110 stars of 
various colors, red, blue, and green, so that looking 
on it, says Herschel, is " like gazing into a casket of 
precious gems." A cluster in Toucan is compact in 
the center, where it is of an orange-red color; the 
exterior is composed of pure white stars, making a 
border of exquisite contrast. 

It is generally conceded that there is some close 

* The process of apparent creation and destruction is going on in the heavens im- 
mediately before the eye of the astronomer. New stars flash light, old stars are lost, 
worlds burst into flame, and their glowing embers fade into darkness. Are they re- 
created into new worlds ? We know not. We only perceive that the same Almighty 
power which fitted up this earth for our home is yet at work among the worlds about 
us, and we are thus witnesses of His eternal presence. 



246 THE SIDEREAL SYSTEM. 

Fig. 96. 




Star-Cluster in Toucan. 



physical relation existing between the stars compos- 
ing such an "archipelago of worlds," but its nature 
is a mystery. They seem generally crowded together 
toward the center, blending into a continuous blaze 
of light. Yet, although they appear so densely com- 
pacted, it is probable that, if we could change our 
stand-point and penetrate one of these groups of 
suns, we should find it, on our approach, opening up 
and spreading out before us, until, in the midst, the 
suns would shine down upon us from the heavens as 
the stars do in our own sky. 

6. Nebulae are faint, misty objects, like specks of 
luminous clouds. A few are visible to the naked eye, 
but the telescope reveals thousands. They differ 
from clusters in not being resolvable into stars when 
viewed through the largest telescopes. With the con- 



NEBULA. 247 

stant improvement made in these instruments, how- 
ever, many so-called nebulae have been resolved, and 
thus the number of clusters has been increased, 
while new nebulae have been discovered. 

Until of late, it was thought that all nebulae were 
simply groups of stars, which would be ultimately 
discerned in the more powerful telescopes yet to be 
made. Spectrum analysis shows, however, that 
many of these luminous clouds are gaseous, and are 
not composed of stars. 

Since all the nebulae maintain the same position 
with respect to the stars, their distance must be 
inconceivably great, and, in order to be visible to us, 
their magnitude must be proportionately vast. They 
are most abundant at the two poles of the Milky 
Way, but are more uniformly distributed over the 
heavens lying near the south pole. 

It is now generally believed that nebulae constitute 
the material for making stars, — are, in fact, sun- 
germs ; that all stars originally existed as nebulae ; 
and that every nebula will, in time, be changed into 
stars. 

Nebulae are divided, according to their form, into 
six classes — elliptic, annular, spiral, planetary , irreg- 
ular nebulce, and nebulous stars, f 

The Elliptic or merely oval nebulae are the most 
abundant. Under this head is classed the Great 
Nebula in Andromeda, which was discovered over 

t This division of the nebula is purely arbitrary, and used only to introduce some 
order of arrangement. The shape of the nebula} changes with the power of the telescope 
through which they are seen. Thus the Great Nebula in Andromeda, as resolved by 
Bond, is no longer oval, but irregular in form. The Ring-Nebula of Lyra, seen through 
the large telescope of to-day, is egg-shaped ; while the Dumb-bell Nebula assumes the 
outline of a chemical retort. 



248 



THE SIDEREAL SYSTEM. 



a thousand years ago, and is visible to the naked 
eye. Prof. Bond, of the Cambridge Observatory, 
Fig. 97. nas Partly resolved it into stars, 

of which he has counted 1500, 
although its nebulous appear- 
ance was still retained. Through 
the telescope, it is one of the 
most glorious objects in the 
heavens. "If we suppose this 
nebula to be one continuous bed 
of stars of different sizes for its 
entire extent, it must comprise 
the enormous number of 30,000,000." 

The distance of such nebulae from the earth passes 
our comprehension. Some astronomers have esti- 
mated that a ray of light would require 800,000 years 
to span the gulf that intervenes. Imagination wearies 
itself in the attempt to understand these figures. 
They teach us something of the limitless expanse of 
that space in which God is working the mysterious 
problem of creation. 




Nebula in Andromeda. 




Nebula in Lyra. 

The Annular Nebulae have the form of a ring. 
There are four of these "ring universes." In the cut 



XEBVLM. no 

is a representation of one in Lyra, — first, as seen by 
Herschel, having in the center a nebulous film like a 

Fig. 99. 




Spiral Cluster in Canes Venatici. 



350 THE SIDEREAL SYSTEM. 

"bit of gauze stretched over a hoop;" second, as 
shown in Lord Rosse's telescope (p. 288), which re- 
solves the filmy parts of the nebula into minute stars, 
and reveals a fringe of stars along the edge. 

The Spiral or Whirlpool Nebula are exceedingly 
curious. The most remarkable one is in Canes Vena- 
tici. It consists of brilliant spirals sweeping outward 
from a central nucleus, and all overspread with a 
multitude of stars.* One is lost in attempting to 
imagine the distance of such a mass, and the forces 
which produce such a "tremendous hurricane of 
matter — perhaps of suns." 

Planetary Nebulae, by their circular form and 
pale, uniform light, resemble the disks of the distant 
Fig. 100. planets of our system. Their 

edges are generally well de- 
fined, though sometimes slightly 
furred. There is one in Ursa 
Major, which, if located at the 
distance of 61 Cygni, would 
"fill a space equal to seven 
times the entire orbit of Nep- 
tune." 

Planetary Nebula. IRREGULAR NEBULiE are those 

which have no definite form. Many present the 
irregularities of clouds torn by the tempest. Some 
of the likenesses which may be traced are strangely 
fantastic : for example, the Dumb-bell Nebula, in the 
constellation Vulpecula, and the Crab Nebula, near 
the southern horn of Taurus. There is also one known 

* Columbus discovered a new continent, and so immortalized his name ; what shall 
we say of the astronomer who discovers a system of worlds ? 




NEBULA. 



251 



as the Great Nebula 
in the Sword-handle 
of Orion, which bears 
a faint resemblance 
to the wings of a bird. 
Nebulous Stars 
are so called because 
they are enveloped 
by a faint nebula, 
usually of a circular 
form. The star is 
generally seen at the 
center, although 
some nebulae sur- 
round tWO Stars, Dumb-bell Nebula. 

having one in each focus. It is thought that these 
may be suns possessing immense atmospheres, which 
are rendered visible somewhat as that of our sun is in 
the zodiacal light ; and that in like manner our sun 
may present to other worlds the appearance of a 
nebulous star.* 
Variable Nebula. — Certain changes take place 




* Nothing in all nature is more suggestive of the magnificence and immensity of 
Creation, than are the nebulous star-clusters, many of which are at such an inconceiv- 
able distance, that the most powerful telescopes show them only as a confused mass of 
light. A casual observer,— even though when led by scientific analogy to resolve each 
little patch of star-dust into a host of separate suns, and to provide each sun with a 
retinue of inhabited planets, — might think of them as little colonies of suns, set on the 
very outskirts of world-creation, and moving in such close proximity, that the peoples 
of the various worlds might communicate with one another. Yet, were he transported 
to some planet whirling about one of those far-off star-suns,— a multitude of which 
blend as a single point of light to our human eyes, — he would see the other suns only as 
fixed stars in the firmament above him ; and though many of them might surpass in 
splendor the glory of our own Sirius, yet all would still remain at such an immense 
distance as to baffle the research of the most powerful telescopic instruments. Thus, too, 
he would probably find each planet revolving at such a distance from its sister planets, 
as to render the certain knowledge of other inhabited worlds as elusive there as here. 



252 



THE SIDEREAL SYSTEM. 

Fig. 102. 




Crab Nebula. 

among the nebulae which can be accounted for only 
under the supposition that they, like some of the 
stars, are variable. Mr. Hind tells us of a nebula in 
Taurus which, in 1852, was distinctly visible with a 
small telescope, but, in 1862, had vanished entirely out 
of the reach of a powerful instrument. The Great 
Nebula in Argo, when observed by Herschel in 1838, 



NEBULAE. 253 

had in the center a vacant space containing a star ot 
the first magnitude, enshrouded by nebulous matter. 
In 1863, the nebulous matter had disappeared, and 
the star was only of the sixth magnitude. These 
facts as yet defy explanation. They illustrate the 
vast and wonderful changes constantly taking place 
in the heavens. 

Double Nebulae. — There seems to be a physical 
connection existing between some of the nebulae, 
similar to that already noticed in respect to certain 
stars. In the case of the latter, this inter-relation 
has been proved, since, even at their distances, their 
movements can yet be traced in the lapse of years. 
" But, owing to the almost infinite depths in the abyss 
of the heavens at which these nebulae exist, thou- 
sands of years, perhaps thousands of centuries, 
would be necessary to reveal any movement." — 
(Guillemin.) 

7. Magellanic Clouds. — Not far from the southern 
pole of the heavens, there are two cloud-like masses, 
distinctly visible to the naked eye, known to navi- 
gators as Cape Clouds. Sir John Herschel describes 
them as consisting of swarms of stars, clusters, and 
nebulae, seemingly grouped together in the wildest 
confusion. In the larger, he found 582 single stars, 
46 clusters, and 281 nebulae. 

8. The Milky Way. — Via Lactea, or the Galaxy, is 
a luminous, cloud-like band that stretches across the; 
heavens in a great circle. It is inclined to the celes- 
tial equator about 63°. This stream of suns is 
divided into two branches from a Centauri to Cyg- 
nus. To the naked eye, it presents merely a diffused 



254 



THE SIDEREAL SYSTEM. 



light ; but with a large telescope it is found to con- 
sist of myriads of stars densely crowded together.* 

These stars are not uniformly distributed through 
the entire extent. In some regions, within the space 
of a single square degree wc can discern as many as 
can be seen with the naked eye in the entire heavens. 
In other parts, there are broad, open spaces. A 
remarkable instance of this occurs nears the South- 
ern Cross. There is a dark, pear-shaped vacancy, 
with a single bright star at the center, glittering on 
the blue background of the sky. In viewing it, one 
is said to be impressed with the idea that he is looking 
through an opening into the starless depths beyond 
the Milky Way. 

The northern galactic pole is situated near Coma 
Berenices, and the southern in Cetus. Advancing 
from either pole toward the Milky Way, the number 
of stars increases, at first slowly and then more 
rapidly, until the proportion at the galaxy itself is 
thirtv-fold. 



HerschcFs Theory of tlie Stellar System. 
* Herschel states that 258,000 stars once passed across the field of his great reflector 
in 41 minutes. With the powerful instruments now making, it is probable that many 
more could be seen. 



KEBtJL^J. 555 

Herschel's Theory.*— Sir W. Herschel has con- 
jectured that the stars are not indifferently scattered 
through space, but are collected in a stratum some- 
thing like that shown in the cut, and that our sun 
occupies a place at S, near where the stream 
branches, A and E being the galactic poles. It is evi- 
dent that, to an eye viewing the stratum of stars in 
the direction SB, SC, or SD, they would seem much 
denser than in the direction SA or SE. Thus are we 
to think of our own sun as a star of the second or 
third magnitude, and of our little solar system as 
plunged far into the midst of this vortex of worlds, a 
mere atom along that 

" Broad and ample road 
Whose dust is gold and pavement stars. " 

9. The Nebular Hypothesis is a theory advanced 
by Laplace, to show how the solar system may have 
been formed, f As since modified, its outlines are as 
follows: In the "beginning," all the matter which 
now composes the sun, and the various planets with 
their moons, was in a gaseous and highly heated 
state. It filled the space at present occupied by the 
system, and extended far beyond the orbit of Nep- 
tune. In other words, the solar system was simply 
an immense nebula. The heat, which is the repel- 

* Other theories have been advanced by astronomers, hut we are as yet ignorant of the 
real structure of the universe outside of our own system. 

t We should remember that this theory aims to tell only the way in which our system 
was developed. The parent nebula must have contained a potential energy equal to all 
the manifestations of force since made in the entire system. Nothing could be devel- 
oped from a mass of nebulous matter the germs of which had not been put in it origin- 
ally by the Creator. The analogies of nature all go to show that the Creator's plan is, 
in general, not to produce any object in a perfect and matured state ; but rather, by a 
gradual growth, to unfold its full form and function. 



256 THE SIDEREAL SYSTEM. 

lant force, overcame the attraction of gravitation. 
Gradually the mass cooled by radiation. As centu- 
ries passed, the repellant force becoming weaker, the 
attractive force drew the matter and condensed it 
toward one or more centers. The nebula then pre- 
sented the appearance of a nebulous star — a nucleus 
enveloped by a gaseous atmosphere. 

According to a well-known law in physics, seen 
in every-day life, wherever matter seeks a center — 
as in a whirlpool, in a whirlwind, or even in water 
poured through a funnel — a rotary motion was estab- 
lished. As the rotary motion of the nebula increased, 
the centrifugal force finally overcame at the exte- 
rior the attraction of gravitation. A ring of condensed 
vapor was then left behind.* Centuries elapsed, and 
again, under the same conditions, a second ring was 
detached. Thus, one by one, concentric rings were 
separated from the parent nebula, all revolving in 
the same plane and in the same direction. These 
different rings, becoming gradually consolidated, 
formed the planets. Generally, however, in this 
process, while still in the vaporous state and slowly 
condensing, the rings themselves detached other 
rings that were in turn consolidated into satellites. 

In the case of Saturn, several oi* these secondary 
rings did not condense into globes, but still remain 
as rings which revolve about the planet, f Mitchell 

*A considerable modification of the Nebular Hypothesis is possible, leaving its 
general idea, however, intact. It is now generally conceded that the several planets 
were not "thrown off," but merely detached and left behind. Proctor thinks that the 
solar system is the result of meteoric aggregation as well as gaseous condensation : the 
planets in their infancy being so large, gathered immense quantities of meteoroids, 
then more abundant than now. 

t In the case of the minor planets and the rings of Saturn, we may suppose that the 



NEBULAR HYPOTHESIS. 257 

naively remarks, " Saturn's rings were left unfinished 
to show us how the world was made." The ring 
which formed the minor planets broke up into small 
fragments, none large enough to attract the rest and 
thus form a single globe. 

The central mass of vapor finally condensed itself 
into the sun, which remains the largest member of 
the system. According to this theory, the sun may 
yet give off a few more planets, whose orbits will not 
exceed its present diameter. After a time, all its 
heat will be radiated into space, its fire will become 
extinct, and life on the planets will cease. We know 
not when this remote event may occur. We can- 
not fathom the purpose of God in creating and main- 
taining this system of worlds, nor can we foretell 
how soon it may complete its mission. We are as- 
sured, however, 

" That nothing walks with aimless feet, 
That not one life shall be destroyed, 
Or cast as rubbish to the void, 
When God hath made the pile complete. " 

In Memoriam. 



rings were composed of matter uniformly distributed ; while in the case of the rings 
that consolidated into planets, there was a nucleus that attracted the rest of the matter 
to itself. It is possible that the rings of Saturn may yet break up and form new satel- 
lites for that planet. Indeed, some hold that one at least of the rings has. thus been 
resolved into small meteorites. These may be attracted, and so picked up, one by one, 
in succession by the larger, until they form another moon, which will continue to re- 
volve about the planet as the ring does now.—" The present state of the solar system is 
a living picture of the entire history of a single planet. From the sun's fire-mist, to 
ring-girt Saturn ; from Saturn, to storm-beaten Jupiter ; from Jupiter, to the sunny 
summer-time of our own planet ; from Earth, to autumn-browned Mars ; and from 
Mars, to the wintry silence and desolation of the dark gulches of the moon, — there is a 
series of stages that carries the thought back into the eternity long passed, as well as 
onward into the measureless depths of the future, and confers upon human intelligence 
a sort of exemption from the limitations of finite existence." — Prof. Winch?ll. 



258 THE SIDEREAL SYSTEM. 



IV. CELESTIAL CHEMISTRY. 

Spectrum Analysis. — The rainbow— that child of 
the sun and shower — is familiar to all. The brilliant 
band of colors, seen when the sunbeam is passed 
through a prism, is scarcely less beautiful. The ray 
of light containing the primary colors is here spread 
out fan-like, and each tint reveals itself. This var- 
iously-colored band is called a spectrum (plural, 
spectra). There are three different kinds of spectra — 

1st. When the light of a solid or liquid body, as 
iron white-hot, is passed through a prism, the spec- 
trum is continuous, and consists of a series of distinct 
colors, varying from red on one side to violet on the 
other. 

2nd. If the light of a burning gas containing any 
volatilized substance be passed through a prism, the 
spectrum is not continuous, but is ornamented by 
bright-colored lines, — sodium giving two yellow lines ; 
strontium, a red one ; silver, two beautiful green ones. 
Each element produces a definite series which can 
be recognized as its test. 

3rd. If a light of the first kind be passed through 
one of the second, the spectrum is crossed by dark 
lines. Thus, if the white light of an electric lamp 
be passed through a flame containing sodium, instead 
of the vivid yellow lines so characteristic of that 
metal, two black lines exactly occupy their place. 
A gaseous flame absorbs the rays of the same color that 
it emits, (See note, p. 310.) 




CD 

O t ' 

B ~ 

° V 



ZZ. CD 
O 

Or cs 

4? IT 



ins 



Rb. 



Li. 



Na. 



Sun. 



Cont. 



CELESTIAL CHEMISTRY. 



259 



The Spectroscope. — This instrument consists of 
two small telescopes, with a prism mounted between 
their object-glasses (Fig. 106). The rays of light enter 
through a narrow slit at A, and are rendered parallel 
by the object-glass. They then pass through the 
prisms at C, are separated into the different colors, 
and, entering the second telescope at D, fall upon the 

Fig. 105. 




eye at B. A third telescope is sometimes attached, 
which contains a minutely-accurate scale for meas- 
uring the distances of the lines. In addition, a 
mirror may throw in at one side of the slit a ray of 
sunlight or starlight, and so we can compare the 
spectrum of the sunbeam with that of any flame we 
desire. 



260 



THE SIDEREAL SYSTEM. 



Eevelations of the Spectroscope Concerning 
the Sun.— The spectrum of the sunbeam is not con- 
tinuous, but is crossed by a large number of dark 
lines, called, from their discoverer, Fraunhofer's 
lines. It is therefore concluded that the sun's light 
is of the third class just named, and that it is pro- 
Fig. 106. 




A Spectruccujje. 

duced by the vivid light of a highly heated body 
shining through a flame full of volatilized sub- 
stances. 

But not only does spectrum analysis thus shed light 
on the physical constitution of the sun, but these 
lines are so distinctive, so marked and varied, that 
the elements of which the sun is composed may be 
discovered.* Thus, for example, iron gives a spec- 
trum of some 450 lines, differing in intensity and 



* The following twenty-two elements have been detected : sodium, calcium, barium. 
magnesium, iron, chromium, nickel, cobalt, hydrogen, manganese, aluminium, titanium, 
palladium, vanadium, molybdenum, strontium, lead, uranium, cerium, strontium, 
cadmium, oxygen, and a probability of several more, such as carbon, silver, tin, etc. 



CELESTIAL CHEMISTRY. 261 

relative length. These are bright when iron vapor 
is burning, and dark when white light is passed 
through such burning vapor. In the solar spectrum 
we have such a coincidence of dark lines, as to make 
the conclusion irresistible that iron is contained in 
the sun's atmosphere.* 

Stars are Suns. — The same method of analysis has 
been applied to the stars. The spectra are marked 
by dark lines. Their constitution is therefore like 
our sun, and they also exhibit familiar elements. 
Betelgeuse, for example, contains many substances 
known to us, but, as is thought, no hydrogen. What 
a world that must be without water ! We thus trace 
in the faintest star that trembles in the measureless 
depths of space, the elements that compose the com- 
mon objects of our own life. We know that we are 
akin to nature everywhere, — a part of a system vast 
as the universe. 

The Motion of a Star may be resolved into two 
components : one representing its motion at right 
angles, and the other its motion parallel to the line 
of vision. The former component can be determined 
by the telescope ; the latter is revealed by the spectro- 

* Recent researches in spectroscopy present important problems. On elevating the 
temperature, it has been found that not only the lines of the spectrum of a substance 
vary, but new ones appear. Certain substances have apparently common lines. A 
molecule containing a few atoms gives a line-spectrum; increase the number of atoms 
and it presents a fluted-spectrum (i. e., one composed of bands, each made up of lines, 
and having a sharp boundary on one side but fading away on the other) ; increase the 
number yet more, and it yields a continuous spectrum. New epieries have therefore - 
arisen in Solar Physics. How many atoms are there really in a specified molecule ? 
What is the. meaning of certain unfamiliar line.; seen in the solar spectrum? Why do we 
not detect in the sun many of those substances that form so large a part of the earth's 
crust ? Lockyer supposes that the so-called elements are really compounds whose mol- 
ecules may be "dissociated" by intense heat, so that in the sun we see only the germs 
of our familiar chemical forms. Read Lockyer's "Spectrum Analysis," and Young's 
" The Sun." 



262 THE SIDEREAL SYSTEM. 

scope. If the star is moving towards us, the number 
of vibrations producing any color will be increased;, 
and hence the dark lines* corresponding to that color 
in the spectrum will be pushed beyond its usual 
place toward the violet end ; if going from us, the 
number will be decreased, and the dark lines be 
pushed toward the red end of the spectrum.* The 
amount of displacement once determined, the velo- 
city of the star can be reckoned by means of well- 
known laws of optics. 

Spectra of Nebulae. — Instead of being marked 
with dark lines, as are the spectra of the stars, many 
of the nebulae exhibit bright lines. Their spectra 
are, therefore, of the 2nd kind. This proves such 
nebulae to consist, not, like the stars, of an intensely- 
heated nucleus shining through a luminous atmos- 
phere, but of a glowing mass of gas.f Out of 60 
nebulae examined by Mr. Huggins, 20 exhibited the 
bright lines belonging to the gases, and all contained 
nitrogen. 

The Solar Flames, which were formerly seen only 
during an eclipse, can now be examined by means of 
the spectroscope, at any time. J The sun has thus 

* The same result is produced in the case of sound. The -whistle of an approaching 
train sounds shriller than when it is reeeeding. See Physics, p. 33P». 

f The Dumb-bell nebula is said to emit a light only about one twenty-thousandth 
part that of a common wax-candle. If this matter be a " sun-germ," how immensely 
must it become condensed before its rushlight glimmering can rival the dazzling bril- 
liancy of even our own sun ! 

I " The red portion of the spectrum will stretch athwart the field of view like a scarlet 
ribbon with a darkish band across it ; and in that band will appear the prominences., 
like scarlet clouds, so like our own terrestrial clouds, indeed, in form and texture, that 
the resemblance is quite startling. One might almost think he was looking out 
through a partly-opened door upon a sunset sky, except that there is no variety or con- 
trast of color ; all the cloudlets are of the same pure scarlet hue. Along the edge of the 
opening is seen the chromosphere, more brilliant than the clouds which rise from it or 
float above it, and, for the most part, made up of minute tongues and filaments." — Young. 



TIME. 263 

been found to be a sea of fire swept by the most vio- 
lent storms. * Flames travel over its surface with a 
velocity of which we can form no conception ; " one 
jet shot out 80,000 miles and disappeared in ten min- 
utes.* 7 Young describes a protuberance that reached 
the enormous height of 350,000 miles and then faded 
entirely away, all within two hours. 



V. TIME. 



Sidereal Time. — A sidereal clay is the exact interval 
of time in which the earth rotates on its axis. It is 
found by marking two successive passages of a star 
across the meridian of any place. This is so abso- 
lutely uniform, that, as recent investigations seem 
to show, the length of the sidereal day has not varied 
more than -fe of a second in 2,400 years, (note, p. 89). 

The sidereal day is divided into twenty-four equal 
portions, which are called sidereal hours, and each 
of these hours into sixty portions, termed sidereal 
minutes, etc. 

Astronomical clocks are regulated to keep si- 
dereal time. The day commences when the vernal 
equinox is on the meridian. Therefore, the time by 
a sidereal clock does not point out the hour of the 
ordinary day. It indicates only how long it is since 
the vernal equinox crossed the meridian, and thus 
shows the right ascension of any star which may 

* Such a storm " coming down upon us from the north would in 30 seconds after it 
had crossed the St. Lawrence be in the Gulf of Mexico, carrying with it the whole sur 
face of the continent in a mass, not of ruin simply, bat of glowing vapor, in which the 
vapors arising from the dissolution of the materials composing the cities of Boston, New 
York, and Chicago would be mixed in a single uadistinguiahable cloud."— Newcomb. 



264 THE SIDEKEAL SYSTEM. 

happen to be on the meridian at that moment. The 
hours of the clock are easily reduced to degrees (p. 
28). The astronomer always reckons the hour of the 
day consecutively up to twenty-four. 

Solar Time. — A solar day is the interval between 
two successive passages of the sun across the meri- 
dian of any place. If the earth were stationary in 
its orbit, the solar day would be of the same length 
as the sidereal ; but, while the earth is turning 
around on its axis, it is going forward at the rate of 
360° in a year, or about 1° per day. When the earth 
has made a complete rotation, it must therefore 
perform a part of another rotation through this 
additional degree, in order to bring the same meri- 
dian vertically under the sun. 

One degree of diurnal rotation is equal to about 
four minutes of time. Hence the solar day is four 
minutes longer than the sidereal day. For the con- 
venience of society, it is customary to call the solar 
day 24 hours long, and make the sidereal day only 
23 hr. 56 min. 4 sec. in length, expressed in mean 
solar time. A sidereal day being shorter than a 
solar one, the sidereal hours, minutes, etc., are 
shorter than the solar ; 24 hours of mean solar time 
being equal to 24 hr. 3 min. 56 sec. of sidereal time. 

From what has been said, it follows that the earth 
makes 366 rotations around its axis in 365 solar days. 

Mean Solar Time. — The solar days are of unequal 
length. To obviate this difficulty, astronomers sup- 
pose a mean sun moving through the equator of the 
heavens (which is a circle and not an ellipse) with a 
perfectly uniform motion. When this mean sun 



TIME. 2G5 

passes the meridian of any place, it is mean noon ; 
and when the true sun is in the same position, it is 
apparent noon. This mean day is the average length 
of the solar days in the year. The clocks in common 
use are regulated to keep mean time. ::: "When it is 
twelve by the clock, the sun may be either a little 
past or a little behind the meridian. 

The difference between sun-time (apparent solar- 
time) and clock-time (mean time) is called the 
"Equation of time:' This is the greatest about the 
first of November, when the sun is over sixteen and 
a quarter minutes in advance of the clock. The sun is 
the slowest about February 10th, when it is about 
fourteen and a half minutes behind mean time. 

Mean and apparent time coincide four times in the 
year — namely, April 15th, June 14th, September 1st, 
and December 24th. On these days, the noon-mark 
on the sun-dial coincides with twelve o'clock. 

The Sun-Bial. — The apparent time of the dial may 
be readily changed to mean time, by adding or sub- 
tracting the number of minutes given in the almanac 
for each day in the year, under the heading "sun 
slow" or "sun fast." A noon-mark is thus a very 
convenient method of regulating a timepiece. \ 

* In France, until 1816, apparent time was used ; and the confusion was so great, 
that Arago relates how the town clocks would differ thirty minutes in striking the same 
hour. As the time varied every day, no watchmaker could regulate a watch or clock to 
keep it. 

t The following manner of obtaining one without a transit instrument may be use- 
ful. Select a level hard surface which is exposed to the sun from about 9 a. m. to i p. m. 
Upon this carefully describe, with compasses, a circle of eight or ten inches in diameter. 
Take a piece of heavy wire, six or eight inches in length, one end of which is sharpened. 
Drive this -perjicndicidarly into the center of the circle, leaving it just high enough to 
allow the extreme end of its shadow to fall upon the circle about 9 \ or 10 a. m. Mark 
this point, and also the place where the shadow touches the circle in the afternoon. 
Take a point half-way between the two, and, drawing a line from that to the center of 
the cire'e, it will be the meridian line, or noon-mark. 



266 



THE SIDEREAL SYSTEM. 



Why the Solar Days are ofUnequal Length.— There 
are two reasons for this, — the unequal orbital motion 
of the earth, and the obliquity of the ecliptic. First : 
the orbit of the earth is an ellipse ; and thus the ap- 
parent yearly motion of the sun along the ecliptic is 
variable. In perihelion, in January, the sun appears 
to move eastward daily 1° 1' 9". 9 ; while at aphelion, 
in July, only 57' 11". 5. As the earth in its diurnal 
motion rotates uniformly from west to east, and the 
sun passes eastward irregularly, this must produce a 
corresponding variation in the length of the solar 
day. The sun, therefore, comes to the meridian 
sometimes earlier and sometimes later than the 
mean noon, and they agree only at perihelion and 
aphelion. 

Second : as we have just seen, the mean sun is sup- 
posed to move in a circle and not an ellipse. This 

would make the 
motion along 
the ecliptic uni- 
form, but the 
obliquity of the 
ecliptic would 
still cause an ir- 
regularity in the 
length of the 
day. The mean 
sun is therefore supposed to pass along the equi- 
noctial, which is perpendicular to the earth's axis ; 
while the ecliptic is inclined to it 23° 27'. Let A 
represent the vernal equinox ; I, the autumnal ; AEI, 
the ecliptic ; AI, the equinoctial ; PK, PL, PM, etc., 




TIME. 2G? 

meridians. Let the distances AB, BC, CD, etc., be 
equal arcs of the ecliptic, which are passed over by the 
sun in equal times. !Next, on the equinoctial, mark 
off distances Aa, cib, be, etc., equal to AB, BC, etc. 
These are equal arcs of right ascension, or hour- 
circles, through which the earth, rotating from 
west to east, passes in equal times. ISTow, meridians 
drawn through these divisions, would not agree with 
those drawn through equal divisions on the ecliptic. 
Hence, a sun moving along the ecliptic, which is in- 
clined, would not make equal days, even though the 
ecliptic were a perfect circle. 

Let us see how the mean and apparent solar days 
would compare. Let the real sun pass in its east- 
ward course from A to B in a certain time ; the mean 
sun moving the same distance would reach the point 
a, since the latter travels on the base and the former 
the hypothenuse of a triangle. The earth, rotating 
from west to east, would cause the real sun to cross 
any meridian earlier than the mean sun ; hence, ap- 
parent time would be faster than clock-time. By 
holding the figure up above us toward the heavens, 
we can see how a westerly sun would cross the meri- 
dian earlier than an easterly one. Following the 
same reasoning, we can see that at the solstice, solar 
and mean time would agree ; while beyond that point 
the mean time would be faster. 

The Civil Day is the mean solar day. It extends . 
from midnight to midnight." The method of divid- 

* Until recently, very many nations terminated one day and commenced the next at 
sunset. Under this plan, 10 o'clock on one day would not mean the same as 10 o'clock 
on another day. The Puritans commenced the day at p. m. The Babylonians, Per- 
sians, and Assyrians began the day at sunrise. 



268 THE SIDEREAL SYSTEM. 

ing the day into two portions of twelve hours each, 
is said to have been adopted by Hipparchus, 150 
years b. c, and is now in use over the civilized 
world. The astronomical method of reckoning the 
hours consecutively up to twenty-four is much more 
convenient, and is therefore coming into general 
favor. The names of the days are derived as follows : 

1. Dies Sqlis Latin .... Sun's day. 

2. Dies Lunse. " ... Moon's day. 

3. Tius daeg. Saxon . . . .Tius's day. 

4. Wodnes daeg... " . Woden's day. 

5. Thurnes daeg.. " . . . Thor's day. 

6. Friges daeg " ... .Friga's day. 

7. Dies Saturni.... Latin . . . .Saturn's day. 

The Tear. — The sidereal year is the interval of a 
complete revolution of the earth about the sun, meas- 
ured by a fixed star. It comprises 365 d., 6 hr., 9 
min., 9.6 sec. of mean solar time. The mean solar 
year (tropical year) is the interval between two suc- 
cessive passages of the sun through the vernal equi- 
nox. It comprises 365 d., 5 hr. ? 48 min., 46.7 see. If 
the equinoxes were stationary, there would be no 
difference between the sidereal and the tropical year. 
As the equinoxes retrograde along the ecliptic 50" of 
space annually, the former is 20 min., 20 sec. longer. 

The anomalistic year is the interval between two 
successive passages of the earth through its perihe- 
lion, which moves eastward about 11". 8 annually. It 
is 4 min., 40 sec. longer than the sidereal year. 

The Ancient Year. — The ancients ascertained the 
length of the year by means of the gnomon. This 
was a perpendicular rod standing on a smooth piano 



TIME. 269 

on which was a, meridian line. When the shadow 
cast on this line was the shortest, it indicated the 
summer solstice; and when it was the longest, the 
winter solstice. The number of days required for 
the sun to pass from one solstice back to it again de- 
termined the length of the year. This they found to 
be 365 days. As that is nearly six hours less than 
the true solar year, dates were soon thrown into con- 
fusion. If, at a certain date, the summer solstice 
occurred on June 20th, in four years it would fall 
on the 21st; and thus it would gain one day every 
four years, until in time the summer solstice would 
happen in the winter months. 

Julian Calendar. — Julius Caesar first attempted to 
make the calendar year coincide with the motions of 
the sun. By the aid of Sosigenes, an Egyptian 
astronomer, he devised a plan of introducing every 
fourth year a leap-year, which should contain an 
extra day. This was termed a bissextile year, since 
the sixth (sextilis) day before the kalends (first day) 
of March was then counted twice. 

Gregorian Calendar. — Though the Julian calen- 
dar was nearly perfect, it was yet somewhat defec- 
tive. It considered the year to consist of 365£ days, 
which is 11 minutes in excess. This accumulated 
year by year, until in 1582 the difference amounted 
to ten days. In that year, the vernal equinox oc- 
curred on the 11th of March, instead of the 21st. 
Pope Gregory undertook to reform the anomaly, by 
dropping ten days from the calendar and ordering 
that thereafter only centennial years which are 
divisible by 400 should be leap-years. The Gregorian 



270 THE SIDEREAL SYSTEM. 

calendar was generally adopted in Catholic countries. 
Protestant England did not accept the change unti] 
1752. The difference had then amounted to 11 days. 
These were suppressed and the 3rd of September was 
styled the 14th.* Dates reckoned according to the 
Julian calendar are termed Old Style (O.S.); and 
those according to the Gregorian calendar, New 
Style (N.S.). 

Commencement of the Tear. — The Jews began 
their civil year with the autumnal equinox ; but 
their ecclesiastical year, with the vernal equinox. 
When Caesar revised the calendar, the Eomans com- 
menced the year with the winter solstice (Dec. 22), 
and it is probable he did not intend to change it 
materially. He ordered it to date from January 1, in 
order that the first year of his new calendar should 
begin with the day of the new moon immediately 
succeding the winter solstice. 

The Earth our Timepiece. — The measure of time 
is, as we have just seen, the length of the mean 
day. This is estimated from the length of the 
sidereal day. Hence, the standard for time is the 
rotation of the earth on its axis. All weights and 
measures are based on time. An ounce is the weight 
of a given bulk of distilled water. This is measured 

* This sweeping change was received in England with great dissatisfaction. Prof 
De Morgan narrates the following : " A worthy couple in a country town, scandalized by 
the change of the calendar, continued for many years to attempt the observance of 
Good Friday on the old day. To this end they walked seriously and in full dress to the 
church door, on which the gentleman rapped with his stick. On finding no admittance, 
they walked as seriously back again and read the service at home. There was a wide- 
spread superstition that, when Cliristmas day began, the cattle fell on their knees in their 
stables. It was asserted that, refusing to change, they continued their prostrations 
according to the Old Style. In England, the members of the Government were mobbed 
in the streets by the crowd, which demanded the eleven days of which they had been 
illegally deprived." 



CELESTIAL MEASUREMENTS. 271 

by cubic inches. The inch is a definite part of the 
length of a pendulum which vibrates seconds in the 
latitude of London. Arago remarks, a man would 
be considered a maniac who should speak of the in- 
fluence of Jupiter's moons on the cotton trade. Yet 
there is a connection between these incongruous 
ideas. The navigator, travelling the waste of waters 
where there are no paths and no guide-boards, may 
reckon his longitude by the eclipses of Jupiter's 
moons, and so decide the fate of his voyage. We 
can easily see how the rotation of the earth on its 
axis influences the cost of a cup of tea. 



VI. CELESTIAL MEASURE- 
MENTS. 

Many persons read the enormous figures which 
indicate the distances and dimensions of the heaven- 
ly bodies with a questioning, indefinite idea, entirely 
unlike the feeling of certainty with which they read 
of the distance between two cities, or the number 
of square miles in a certain State. Many, too, imag- 
ine that celestial measurements are so mysterious in 
themselves that no common mind can hope to grasp 
the methods. Let us attempt the solution of a few 
of these problems. 

1st. To Find the Distances of the Planets from 
the Sun.— In Fig. 108, E represents the earth; ES, 
the earth's distance from the sun; V, the planet 
Venus; and VES, the angle of elongation (a right- 
angled triangle). It is clear that,. as Venus swings 



272 



THE SIDEREAL SYSTEM. 




apparently east and west of the sun, this angle may 
be easily measured ; also, that it will be the greatest 
when Venus is in aphelion and the earth in peri- 
helion at the same time, for then VS will be the 
longest and ES the shortest. Now in every right- 
ly. io8. angled triangle the propor- 
tion between the hypothe- 
nuse, ES, and the side op- 
posite, VS, changes as the 
angle at E varies, but with 
the same angle remains the 
same whatever may be the 
length of the lines them- 
selves. This proportion be- 
tween the hypothenuse and 
the side opposite any angle 
is termed the sine of that 
angle. Tables are published containing the sines for 
all angles. In this way, the mean distance of Venus 
is found to be T 7 o 2 o that of the earth; Mars, i times; 
Jupiter, 5| times, etc. * 

2nd, To Measure the Moon's Distance from the 
Earth. — (1.) The Ancient Method.— As the moon's 
distance is so much less than that of the other 
heavenly bodies, it is measured by the earth's semi- 
diameter. The method, an extremely rough one, 
which was in use among the ancients, was something 

* If the pupil has studied Trigonometry, he may apply here the simple proportion — 
ES : VS : : Radius : Sine of 47° 15" = greatest elongation of Venus 

The same result would be obtained by the use of Kepler's third law ; and on page 
19, we saw how the distances of the planets themselves could be determined by the 
periodic times, if the distance of the earth from the sun is first known. So that when 
we have accurately determined the sun's distance from us, we can then decide by either 
of the methods named the distance of all the planets. Indeed the sun's distance is, as 
already remarked, the "foot-rule" for measuring all celestial distances. 



Comparative Distance of Venus and the 
Earth. 



CELESTIAL MEASUREMENTS. 273 

like the following : In an eclipse of the moon, that 
body passes through the earth's shadow in about 
four hours. If, then, in four hours, the moon travels 
along its orbit a distance equal to the diameter 
of the earth, in twenty-four hours it would pass over 
six times, and in a lunar month (about thirty days) 
one hundred and eighty times, that distance. The 
circumference of the lunar orbit, then, must be one 
hundred and eighty times the diameter of the earth. 
The ancients supposed the heavenly orbits to be 
circles, and, as the diameter of a circle is about i of 
the circumference, they deduced the diameter of the 
moon's orbit as 120 times, and the distance of the 
moon from the earth as 60 times, the semi-diameter 
of the earth. 

(2.) Modern Method by the Lunar Parallax. — 
Under the head of parallax, we saw how, in common 
life, we obtain a correct idea of the distance of an 
object by means of our two eyes. We proved that 
one eye alone gives no notion of distance. Just, 
then, as we use two eyes to find how far from us an 
object is, so the astronomer uses two astronomical 
eyes, or observatories, located as far apart as pos- 
sible, to find the parallax of a heavenly body. In 
Fig. 109, M represents the moon ; G, an observatory 
at Greenwich ; and C, another at the Cape of Good 
Hope. At the former, the distance from the north 
pole to the center of the moon, measured on a 
meridian of the celestial sphere, is found to be 108°. 
At the latter station, the distance from the south 
pole to the moon's center is measured in the same 
way, and found to be 734°.. The sum of these angles 



274 THE SIDEREAL SYSTEM. 

is 181 i°. How, tho entire distance from the north 
pole around to the south pole, measured on a meridian, 
can be only half a great circle, cr 180°. This differ- 
ence of 1J° must be tho difference in the position of 
the moon, as seen from the two observatories. For 
the observer at the former station will see the moon 
projected on the celestial sphere at G', and in meas- 
uring its distance from the north pole will measure 
an arc bQ' further than if he were located at E, the 
center of the earth. The observer at the latter sta- 
tion will see the moon projected on the celestial sphere 
at C, and in measuring its distance from the south 
polo will measure an arc bC more than if he were 
located at E, the center of the earth. The sum of 
6G' and bC = G'C is the difference in the position of 
the moon as seen from the two stations. In other 
words, it is the moon's parallax. The arc G'C, 
measures the angle C'MG'; that angle is equal to the 
opposite angle GMC = li°. How, in the four-sided 
figure GECM, the sides GE and CE are both equal 
radii of the earth = 3,95G miles; while the distance 
from G to C is the difference in the latitude of the 
two places. ,The angles ZGM and Z'CM, being the 
zenith distances of the moon, are known, and so the 
angles MGE and MCE are easily found. EM, tho 
moon's distance from the center of the earth, is 
thus readily computed by a simple trigonometrical 
formula. 

(3.) The Horizontal Parallax of the Moon is 
most commonly found by estimating its distance, 
not from the north and south poles, as just explained 
under the general meaning of the term parallax, but 



CELESTIAL MEASUREMENTS. 



from a fixed star. The moon's horizontal parallax is 
now estimated at 57', which makes its distance about 
sixty times the earth's semi-diameter (p. 273).* 



Fig. 100. 




P' z 

Measuring Moon's Distance from the Earth. 

3. To Find the Sun's Distance from the Earth. — 
This might be estimated by obtaining the solar paral- 
lax in the same manner as the lunar parallax. It 
would be necessary only to take the sun's distance 
from the north and south poles respectively at Green- 
wich and the Cape of Good Hope, and then subtract- 
ing 180° from the sum of the two angular distances, 
the remainder would be the solar parallax. The dif- 
ficulty in this method lies in the fact that when the 
sun shines the air is full of tremulous motion. This 
increases refraction — that plague of all astronomical 
calculations — to such an extent that it becomes im- 

* In figure 110, let S represent the moon, sun, or any other heavenly body ; AB, the 
semi-diameter of the earth ; and ASB, the " horizontal parallax " of the body. Then, by 
the following trigonometrical formula, the distance from the eajtli may be easily calcu- 
lated— A3 : AB : : Radius : Ein-of A&B _..•... 



276 THE SIDEREAL SYSTEM. 

possible to calculate so small an angle with any 
accuracy. Neither can the parallax be estimated, as 
in the case of the moon, by measuring the distance 




from a fixed star, since when the sun shines the stars 
near by are invisible even in a telescope. Astrono- 
mers have therefore been compelled to resort to other 
methods. 

(1.) Calculation of Solar Parallax by Observ- 
ing Mars. — We have already seen that the distance 
of Mars from the sun is | that of the earth from the 
sun. If, therefore, we can find Mars's distance from 
the earth, we can multiply it by three, and so obtain 
the distance of the sun from the earth. In 1862, when 
Mars was in opposition, it came very near us, for it 
was in perihelion while the earth was in aphelion, 
so that its distance (as since ascertained) was only 
about 34,000,000 miles. Astronomers at Greenwich 
and the Cape, and at various American and European 
observatories, calculated the distance of the planet 
from the north and south poles, as well as from seve- 
ral fixed stars, in the manner just explained for 
obtaining the lunar parallax. The result of these 
observations fixed the solar parallax at 8". 94,* mak- 
ing the sun's distance 91,430,000 miles. 

* By the formula on page 275, \vc have, AS : AB : : Radius : Sin 8".94. 



celestial measurements. 277 

(2.) Calculation of Solar Parallax by Obser- 
vation of the Transit of Venus. — In the figure, let 
A and B represent the position of two observers sta- 

Fig. 111. 



Transit of Venus. 

tioned at opposite sides of the earth. At the time of 
the transit, the one at A will see the planet Venus as 
a round black spot at V" on the sun's disk, while the 
one at B will see it at V. The distance V'V" is the 
difference in the position of Venus as seen from the 
two stations on the earth. The distance AB is the 
diameter of the earth. The distance V'V" is as much 
greater than AB as VV" is greater than VA. The 
distance of Venus from the sun is known, by Prob. 
I., to be .72 that of the earth. The distance of Venus 
from the earth must, then, be 1.00— .72 =.28. Hence, 
VV", the distance from the sun to Venus, = .72 -=-. 28= 
2.5 times the length of AV, the distance of Venus from 
the earth. Therefore, V'V" is equal to %\ times AB, 
the earth's diameter, or 5 times the solar paral- 
lax. Knowing the hourly motion of Venus, it is 
necessary only for each observer to find when the 
planet's disk enters upon and leaves the sun's disk, 
to determine the length of the path (chord) it 
traces. A comparison of the length and direction 



278 THE SIDEREAL SYSTEM. 

of these chords will give the length V V" in seconds 
of space. 

The advantage of this method is that, as the dis- 
tance V Y" is two and half times that of AB, an 
error in measuring that chord affects the solar par- 
allax less than one-fifth. 

Time of a Transit of Yenus. * — This is an event of 
rare occurrence. It happens only at intervals of 8, 
105 \ ; 8, 121 J, years, &c. Were the planet's orbit in 
the same plane as the ecliptic, a transit would take 
place during each synodic revolution ; but as it is 
inclined about 3|°, the transit can occur only when 
the earth is at or near one of the nodes at the same 
time with the planet when in inferior conjunction. 
As the nodes of Venus now fall in that part of the 
earth's orbit which we pass in the beginning of 
June and December, transits always occur in those 
months. 

The Transit of June 3rd, 1769, excited great inter- 
est. King George III. fitted out an expedition to 
Tahiti, under the command of the celebrated naviga- 
tor, Capt. James Cook. In order to make the angle 
as great as possible, and so increase the length of the 
chords, or paths of the planet across the sun, astron- 
omers were sent to all the most favorable points of 
observation— St. Petersburg, Pekin, Lapland, Cali- 

* The first transit ever seen was witnessed by Horrox, a young amateur astronomer 
residing near Liverpool. His calculations fixed upon Sunday, Nov. 24, 1639 (0. S.). 
He, however, commenced his watch of the sun on Saturday preceding. The following 
day he resumed his observation at sunrise. The hour for church arriving, he repaired 
to service as usual. Returning to his labor immediately afterward, he says : "At this 
time an opening in the clouds, which rendered the sun distinctly visible, seemed as if 
Divine Providence encouraged my aspirations ; when— oh most gratifying spectacle ! the 
object of so many earnest wishes— I perceived a new spot of perfectly round form that 
had just entered upon the left limb of the sun." 



CELESTIAL MEASUREMENTS. 279 

fornia, etc. They fixed the solar parallax at 8". 58, 
making the sun's distance 95,293,055 miles.* 

The transits of Dec. 8, 1874, and Dec. 6, 1882, were 
carefully observed by several government expedi- 
tions ; the results have not yet been fully announced. 

The next transits will happen, 

June 8 2004. 

June 6 2012. 

December 11 2117. 

December 8 2125. 

June 11 2247. 

The transits of Mercury are more frequent; but 
owing to the nearness of the planet to the sun, they 
are of little value in determining the solar parallax. 

Changes in the Estimate of the Solar Paral- 
lax. — About 1824, Encke deduced 8". 58 as the prob- 
able result of the observations upon the transit of 
1769. This conclusion held the ground for nearly 
thirty years, and the corresponding solar distance of 
95,293,000 miles is found in all the older text-books. 
About 1860, Le Verrier announced that he could 
reconcile the theories regarding certain of the plan- 
ets only by assuming a greater solar parallax. As 
the result of various calculations, together with the 
material furnished by the observations upon the 

* Le Gentil, sent out by the French Academy to observe the transit of 1761 in the 
East Indies, was prevented from making his first port by the war with England. High 
winds afterward kept him out at sea till the transit was over. He then resolved to re- 
main abroad until after the transit of 1769. Eight long years passed, and the morning 
of June 3, 1769, dawned bright and beautiful. Le Gentil, with his instruments all in 
place, was counting the moments for the long-awaited transit to begin ; when, suddenly, 
the sky grew black with clouds, and a tropical storm, the first in days, swept by. 
Meantime, Venus came and went, and the ill-fated Le Gentil had again lost the oppor- 
tunity of years. Prostrated by his bitter disappointment, it was two weeks before he 
could hold his pen to write the story of his second failure. 



280 THE SIDEBEAL SYSTEM. 

planet Mars in 1862, a new parallax of 8". 94 was 
obtained. This has been accepted by all until 
recently, and was used in former editions of this 
work. It is now known to be too large, and astron- 
omers are making every effort to determine this most 
important factor in celestial measurements. As al- 
ready stated on page 36, the parallax at present re- 
ceived is about 8*. 80, which represents a mean solar 
distance of 92,885,000; in round numbers, 93,000,000, 
as given in the present edition. 

The difficulty of determining the solar parallax 
accurately will be seen, when one is told that the 
correction from the old value of 8". 58 to the recent 
one of 8". 94, was a change in the angle equal to that 
which the breadth of a human hair would make 
when seen at a distance of 125 feet. Yet this reduced 
the estimated distance of the sun from 95,293,000 
miles, to 91,430,000 miles. 

4. To Find tho Longitude of a Place.— (1.) The 
Solar Method. — If the sailor can see the sun, he 
watches it closely with his sextant ; and when the 
sun ceases to rise any higher in the heavens it is ap- 
parent noon. By adding or subtracting the equation 
of time (as given in his almanac), he obtains the true 
or mean noon. Ho then compares the local time thus 
determined, with the Greenwich time as kept by the 

* It is pleasant to notice that fie astronomer can 2iredlct with the utmost precision. 
He announces that on such a year, mouth, day, hour, and second, a celestial body will 
occupy a certain position in the heavens. At the time indicated, we point our telescope 
to the place, and, at the instant, true beyond the accuracy of any- timepiece, the orb 
sweeps into view ! A prediction of the Nautical Almanac is received with as much 
confidence as if it were a fact contained in a book of history. " On the trackless ocean, 
this book is the mariner's trusted friend and counsellor; dady and nightly its revela- 
tions bring safety to ships in all parts of the world. It is something more than a mere 
book. It is an ever-present manifestation of the order and harmony of the universe." 



CELESTIAL MEASUREMENTS. 281 

ship's chronometer. The difference in time reduced 
to degrees, gives the longitude. 

(2.) The Lunar Method. — On account of the diffi- 
culty in obtaining a watch which will keep the exact 
Greenwich time through a long voyage, the moon is 
more generally relied upon than the chronometer. 
The Nautical Almanac* i& always published, for the 
benefit of sailors, three years in -advance. It gives 
the distance of the moon from the principal fixed 
stars which lie along its path, at every hour in the 
night. The sailor has only to determine with his 
sextant the moon's distance from any fixed star, and 
then, by referring to his almanac, find the correspond- 
ing Greenwich time. By comparing this with the 
local time, and reducing the difference to degrees, 
etc., he obtains the longitude. 

5. To Find the Latitude of a Place.— (1.) By 
means of the sextant find the elevation of the pole 
above the horizon, and this gives the latitude direct- 
ly. (Fig. 35.) 

(2.) In the same manner, determine the height of 
the sun above the horizon at noon. The sun's decli- 
nation for that day (as laid down in the almanac), 
added to or subtracted from this, gives the height of 
the equinoctial above the horizon. Subtract this 
result from 90°, and the remainder is the latitude. 

"Place an Astronomer on board a ship ; blindfold him ; carry him by 
any route to any ocean on the globe, whether under the tropics or in one 
of the frigid zones ; land him on the wildest rock that can be found ; 
remove his bandage, and give him a chronometer regulated to Greenwich 
or Washington time, a transit instrument with the proper appliances, and 
the necessary books and tables, and in a single clear night he can tell his 
position within a hundred yards by observations oj^hc stars." 



2$2 THE SIDEREAL SYSTEM. 

6. To Find the Circumference of the Earth. — If 

the earth were a perfect sphere, it is obvious that 
degrees of latitude would be of the same length 
wherever measured on its surface. Each would be 
^o of the entire circumference. If, however, a per- 
son sets out from the equator, and travels along a 
meridian toward either pole, and, when the polar star 
has risen in the heavens one degree above the horizon, 
he marks the spot, and then continues his journey, 
marking each degree in succession, he will find that 
the degrees are not of equal length, but increase 
gradually from the equator to the pole. If, now, the 
length of a degree be measured at different places, 
the rate of variation can be found, and then the 
average length be estimated. Measurements for this 
purpose have been made in Peru (almost exactly at 
the earth's equator), Lapland, England, France, 
India, Russia, etc. So great accuracy has been at- 
tained, that Airy and Bessel, who have solved the 
problem independently, differ in their estimate of 
the equatorial diameter but 77 yards, or only y^fg- of 
a mile. 

7. To Find the Relative Size of the Planets. — The 
volumes of two globes are proportional to the cubes 
of their like dimensions. The diameter of Mercury 
is 3,000 miles, and that of the earth 7,925 ; then, 

The volume of Mercury : the volume of the earth : : 3000 3 : 7925 3 . 

The same principle applied to the volume or bulk of 
the sun gives — 

The bulk of the sun : bulk of the earth :: 866,000 s : 7925 s . 



PRACTICAL QUESTIONS. 283 

8. To Find the Diameter of the Sun. — (1.) A very 
simple method is to hold up a circular piece of paper 
before the eye at such a distance as exactly to hide 
the entire disk of the sun. Then we have the pro- 
portion, 

As dist. of paper disk : dist. of sun's disk : : diam. of paper d. : diam. sun's d. 

(2.) The apparent diameter of the sun, as seen 
from the earth, is about 32': the apparent diameter 
of the earth, as seen from the sun, is twice the solar 
parallax, or 17". 60 (p. 36). Thence, the 

Ap. diam. of earth : ap. diam. of sun : : real diam. of earth : real diam. of sun. 

(3.) Knowing the apparent diameter of the sun, 
and its distance from the earth, the real diameter is 
found by Trigonometry. In figure 110, let S represent 
the earth ; AB, the radius of the sun ; and ASB, half 
the apparent diameter of the sun. We shall then 
have the proportion, 

AS : AB : : radius : sin. 16' (half mean diam. of sun). 

By a similar method the diameters of the planets are 
obtained. 



PRACTICAL QUESTIONS. 

1. In what constellation is Job's Coffin ? The Letter Y ? The Scalene 
Triangle ? The Dipper ? The Kids ? The Triangles ? 

2. Name some facts in the solar system for which the nebular hypo- 
thesis fails to account. 

3. Which is probably hotter, a yellow or a red star ? 

4. Are any of the stars likely to collide with each other ? 

5. Is the real day longer or shorter than the apparent one ? 

6. Do we ever see the stars ? 



284 PRACTICAL QUESTIONS. 

7. "What fixed star is nearest the earth ? 

8. How often is Polaris on the meridian of a place ? 

9. How do we know that the stars are suns ? 
10.. Can a watch keep apparent time ? 

11. How could a child be 8 years old before a return of its birthday ? 
12 When will a watch and a sun-dial agree ? 

13. What star will be the Pole Star next after Polaris ? 

14. Why is the birthday of Washington celebrated on Feb. 22, when he 
was born Feb. 11, 1732(0. S.)? 

15. Does the tide have any effect on the length of the day ? 

16. Will the Big Dipper always look as it does now ? 

17. How many times does the earth turn on its axis every year ? 

18. Does the spectroscope tell us anything concerning the constitution 
of the moon, or any of the planets ? 

19. When the United States bought Alaska from Russia, the calendar 
used there was found to be one day ahead of our reckoning. Why was 
this ? 

20. Why do the dates of the solstices and equinoxes vary a day in differ- 
ent years ? 

21. Why are not forenoon and afternoon of the same day, as given in 
the almanac, of equal length ? 

22. In what part of the heavens do the stars apparently move from west 
to east ? 

23. What year was only nine months and six days long ? 

24. What day .will be the last day of the Nineteenth Century ? 

25. If one should watch the sky, on a winter's evening, from 6 p. M. to 
6 a. m. , what portion of the celestial sphere would he be able to see ? 

26. How do we know that the moon has little, if any, atmosphere ? 

27. In Greenland, at what part of the year will the midnight sun be seen 
due north ? 

28. Can you give any other proof of the rotundity of the earth, besides 
that named in the text ? 

29. Point out the error in the following passage from Byron's "Darkness" 

where the poet, in describing the effect of the sun's destruction, says — 

•' I had a dream, * * * 

* * * which was not all a dream, 
The bright sun was extinguished, and the stars 
Did wander darkling in the external space 
Rayless and pathless." 



PRACTICAL QUESTIONS. 285 

30. Explain the remark of the First Carrier in Scene i, Act n, King 
Henry IV : " An't be not four by the day, I'll be hanged : Charles' wain 
is over the new chimney." 

31. Why does not the earth move with equal velocity in all parts of its 
orbit ? 

32. How many Jovian-years old are you ? 

33. Why is the sky blue ? 

34. At what season of the year does Christmas occur in Australia ? 

35. What causes the apparent movement of the sun north and south ? 

36. On what part of the earth is the twilight the longest ? The shortest ? 

37. Name the causes which make our summer longer than winter. 

38. Why is not total darkness produced when a dense cloud passes be- 
tween us and the sun ? 

39. Why does the time of the tide vary each day ? 

40. Why is an annular, longer than a total, eclipse ? 

41. Why is it colder in winter than in summer ? 

42. Do the solar spots affect our weather ? 

43. Can the moon be eclipsed in the day-time ? 

44. Why are the sidereal days of uniform length ? 

45. Why are not the solar days of uniform length ? 

46. What does the moon's phases prove ? 

47. Why do the sun and moon appear flattened when near the hori- 
zon ? 

48. How many stars can we see with the naked eye ? 

49. Is there ever an annular eclipse of the moon ? 

50. "While the sun rises and sets 365 times, a star rises and sets 366 
times." Explain. 

51. How many moons are there in the solar system ? 

52. What causes the twinkling of the stars ? 

53. Name some of the uses of the stars. * 

* "To the astronomer, the fixed stars are immovable boundary-stones by which he 
determines the courses of the wandering heavenly bodies. To the geographer, they are 
the signal-stations according to which he surveys the chart of the earth by the heavens. 
To the mariner, they are the lights that direct him over the dark paths of the seas. To 
the hunter, the herdsman, the wanderer, they are a clock. To the farmer, they are a 
calendar. The historian finds in them many a memorable event in the oldest Grecian 
history. The poet reads in them the charming Grecian mythology, which has furnished 
such rich materials to dramatic art. ; and every person of sensibility receives from them 
an impulse to worship, meditation, and hope." 



£86 PRACTICAL QUESTIONS. 

54. Describe the methods by which we determine the distance of the sun 
from the earth. 

55. Why do not the signs and the constellations of the Zodiac agree ? 

56. When we look at the North Star, how long since the light that enters 
our eye has left that body ? 

57. In what direction does a comet's tail generally point ? 

58. What is the cause of shooting stars ? 

59. Why does the crescent moon appear larger than the dark bod}^ of the 
moon ? 

60. What is the real path of the moon ? 

61. What would be the result if the axis of the earth were parallel to the 
plane of its orbit ? 

62. Do we see the same stars at different seasons of the year ? 

63. Why do we not perceive the earth's motion in space ? 

64. Did the earth ever shine as a star ? Does it now shine as a planet ? 

65. What is the nebular hypothesis ? 

66. What is the cause of the solar spots ? 

67. Would it make the new moon "drier" or "wetter" if the moon's 
path ran north of, instead of on, the ecliptic at the time of new moon ? 

68. Under what conditions are we accustomed to transfer motion ? 

69. Why do not the planets twinkle ? 

70. Why is the horizon a circle ? 

71. What causes are gradually increasing the length of the day ? 

72. What distance does the moon gain in her orbit each year ? 

73. State the general argument which renders it probable that other 
worlds are inhabited. 

74. Illustrate the uniformity of Nature. What thought does this 
suggest ? 

75. At what rate are we traveling through space ? How is this determ- 
ined ? 

76. Why does the length of a degree of latitude increase in going from 
the equator toward either pole of the earth ? 

77. How can you detect the yearly motion of the sun among the stars ? 

78. Have you actually traced the movement of any one of the planets, so 
as to understand its peculiar and irregular wandering among the stars ? 

79. How do you explain the varied aspect of the heavens in the different 
seasons of the year ? 



PRACTICAL QUESTIONS. 287 

80. How does the spinning of a top illustrate the subject of precession ? 

81. Why do solar eclipses come on from the west and cross to the east, 
while lunar eclipses come on from the east and cross to the west ? 

82. Newcomb, in his Astronomy, says that, " If, when the moon is near 
the meridian, an observer could in a moment jump from New York to 
Liverpool, keeping his eye fixed upon that body he could see her apparently 
jump in the opposite direction about the same distance. " Explain. 

83. When, and by whom, was the basis of the calendar we now use fully 
established ? 

84. How much is the Russian reckoning of time behind ours ? 

85. Is there any gain in having the astronomical and the calendar year 
agree ? 

86. What religious festival is fixed each year by the motion of the moon ? 

87. Why can we, at different times, see both poles of the planet Mars ? 

88. What famous astronomical discovery was made on the first day of 
this century ? 

89. Do the stars rise and set at the poles ? 

90. Name and locate the stars of the first magnitude which are seen in 
our sky. 

91. Name three bright stars which lie near the first meridian. 

92. What events were transpiring in our history a Saturnian century ago? 

93. What is the sun's declination at the winter solstice ? At the autumnal 
.equinox ? 

94. Will the width of the terrestrial zones always remain exactly 
as now ? 

95. Is it always noon at 12 o'clock ? 

96. When the sun's declination is 23^° N., in what sign is he then 
located, and what is his R. A. ? 

97. What is the apparent diameter of the sun ? 

98. How can a sailor find his latitude and longitude at sea ? 

99. How many miles on the solar disk represent a second of apparent 
diameter ? 

100. At what latitude will there be twilight during the entire midsummer 
night. 



IV. 



APPENDIX 



Fig. IIS. 




Cambridge Equatorial Telescope. 



APPENDIX, 



TABLE ILLUSTRATING KEPLER'S THIRD LAW. (Chambers.) 

In the first column are the relative distances of 
the planets from the sun ; in the second, the periodic 
times of the planets ; and in the third, the squares 
of the periodic times divided by the cubes of the 
mean distances. The decimal points are omitted in 
the third column for convenience of comparison. 
The want of exact uniformity is doubtless due to 
errors in the observations. 



Vulcan ? 
Mercury 
Venus - 
Earth - 
Mars - 
Jupiter 
Saturn 
Uranus 
Neptune 



.143 


19.7 


132 716 


.38710 


87.969 


133 421 


.72333 


224.701 


133 413 


1. 


365.256 


133 408 


1.52369 


686.979 


133 410 


5.20277 


4,332.585 


133 294 


9.53858 


10,759.220 


133 375 


19.18239 


30,686.821 


133 422 


30.03627 


60,126.722 


133 413 



Arago, speaking of Kepler's Laws, says : " These interesting laws, tested for every 
planet, have been found so perfectly exact, that we do not hesitate to infer the dis- 
tances of the planets from the sun from the duration of their sidereal periods ; and it 
is obvious that this method possesses considerable advantages in point of exactness." 



MEASUREMENTS OF THE EARTH'S DIAMETER, 



Polar diameter 
Equatorial diameter 
Compression - - 




292 



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QUESTIONS FOR CLASS USE. 



These are the questions which the author has used in his own classes 
for review and examination. In the historical portion, he has required 
his pupils to write articles upon the character and life of the various 
persons named, gathering materials from every attainable source. He 
has also introduced whatever problems the class could master, taking 
topics from the article on Celestial Measurements and the various 
mathematical treatises. . 

Introduction. — Define Astronomy. Is the earth a planet ? Is the 
moon a planet? What is the sky? Why does it seem concave ? What 
gives it its color? What is the difference in the appearance of a fixed 
star and a planet? What is the Milky Way? In what direction does 
it span the heavens? In what season of the year is it most brilliant ? 

I. THE HISTORY. 

5-6, What can you say of the antiquity of astronomy? How far 
back do the Chinese records extend ? Name some astronomical phe- 
nomena they contain. Why were these astronomers at fault in failing 
to announce the eclipse ? (Ans. Certain religious ceremonies were 
performed on such occasions, and their omission was believed to expose 
the nation to the anger of the gods.) Why should the Chaldeans have 
become versed in this study? How ancient are their records ? What 
discoveries did they make? How does the Asiatic differ from the Euro- 
pean mind ? 

7. What Grecian philosopher early acquired a reputation in this 
science? What other discovery did Thales make (Physics, p. 251)? 
What did he teach ? What memorable eclipse did he predict? What 
did Anaximander teach? In what century did Pythagoras live? What 
was his characteristic trait ? Did he advance any proof of his system-? 
Explain his theory. How does it differ from ours? What strange 
views did he hold ? 

8. When did Anaxagoras live? What.cfid he teach? What theory 
did Eudoxus advance? What. is the theory of the crystalline spheres? 



294 QUESTIONS FOR CLASS USE. 

What has Hipparchus been styled ? What addition did he make to 
astronomical knowledge? How many stars in our present catalogue 
(p. 207) ? How did Egypt rank in science at an early day ? What 
preparation did the Grecian philosophers make to fit themselves for 
teachers ? How long did Pythagoras travel for this purpose? 

9. What can you say of the School at Alexandria ? What great work 
did Ptolemy write ? What theory did he expound? Was it original? 
What discovery did Eratosthenes make ? Describe that method (p. 282). 
Show how the movements of the planets puzzled the ancients, 

10. What was the theory of " cycles and epicycles " ? Did the 
ancients believe in the reality of this cumbrous machinery? Did this 
theory possess any accuracy ? 

11. Could it be adapted to explain any new motion? What was the 
remark of Alfonso? Describe the progress of learning among the 
Saracens, 

12. Where was the first Observatory in Europe built? When did 
Spain lose her prominence in scientific studies? 

13. What is astrology ? What was its association with astronomy? 
State something of the repute in which astrology was held. Tell what 
you can of the system. What use did it subserve ? 

14. What theory displaced the Ptolemaic? W 7 hen? Was the system 
of Copernicus original ? What credit is due him? Describe his idea 
of apparent motion. How did he apply this to the heavenly bodies ? 
What crudity did he retain ? 

15. Who was Tycho Brahe ? What was his theory? How did it 
differ from Ptolemy's and Copefnicus's ? What good did Brahe 
accomplish? Could he generalize his facts? Had he a telescope? 
How did Kepler differ from Brahe? What were the two prominent 
characteristics of Kepler? 

16-19. State his three laws. Tell how he discovered the first. The 
second. The third. Describe the ellipse. Define focus, perihelion, 
and aphelion. What remarkable statement did Kepler make? W 7 he:i 
did Galileo live ? 

20. What discoveries did he make in Physics? In astronomy? 
What advantage did he have over his predecessors ? Give an account 
of his observations on the moon. On Jupiter's moons. 

21. Why did this settle the controversy between the Ptolemaic and 
the Copernican system ? How were Galileo's discoveries received ? 
Give some of Sizzi's arguments. W T ho discovered the law of gravita- 
tion ? 

23. Repeat it. How was this idea suggested? What familiar laws 
of motion aided Newton?* How did he apply these to the motion of 
the moon ? Repeat the story of his patient triumph. 



THE SOLAR SYSTEM. 295~ 

24, 25. What is the celestial sphere ? Give the two illustrations which 
show its vast distance from the earth. Why can we not see the stars 
by day, as by night? What portion of the sphere is visible to us? 
Name the three systems of circles. 

26-30. Name and define (1) the principal circle, (2) the secondary 
circles, (3) the points, and (4) the measurements of each system. De- 
fine especially, because in common use, zenith, nadir, azimuth, alti- 
tude, equinoctial, right ascension^ declination, equinox, ecliptic, colure, 
and solstice. What is N or S in the heavens? 

31. What is the Zodiac? How wide is it? How ancient? How 
is it divided ? State the names and signs.* State the meaning of each 
U>. 210.) 

II. THE SOLAR SYSTEM. 

What bodies compose the solar system? Describe how we are to 
picture it to ourselves. 

The Sun. — What is its sign? Its distance from us? Illustrate. 
What is the solar parallax (see pp. 121, 275)? What change has recently 
been made in the estimate of the parallax of the sun, and of its distance 
from the earth? (See p. 279.) 

37. How are celestial distances measured ? What is the color of the 
sun ? To what is the sun's light equal ? To how many moons ? 

38. To what is its heat equal ? Illustrate. What proportion of the 
sun's heat reaches the earth? What is the apparent size of the sun? 
How does this vary ? 

39. 40. State the solar dimensions. (1) diameter — illustrate ; (2) 
volume; (3) mass ; (4) weight; (5) density. How large did Pythagoras 
think the sun is? Tell something about the force of gravity on the 
sun. How much would you weigh if carried to its surface? (The force 
of gravity on the sun as compared with the earth is 27.6.) How does 
the sun appear to the naked eye ? 

41. How can wc seethe spots? What were formerly the views of 
astronomers with regard to the sun's face ? 

42. When were the spots discovered ? Tell something about the 
number of the spots. Their location. Size. What number of miles 
subtend a second of arc at the distance of the sun? 

* "The Ram, the Bull, the Heavenly Twins, 
And next the Crab, the Lion shines, 

The Virgin and the Scales, 
The Scorpion, Archer, and He-goat, 
The Man that bears the watering-pot, 

And-Fish with flittering tails." 



296 QUESTIONS FOR CLASS USE. 

43. Describe the parts of which the spots are composed. Describe 
the motion of the spots. 

44. How do the spots change in form as they pass across the disk ? 
What does this prove ? What is the length of a solar axial rotation ? 

• 45, 46. Explain a sidereal and a synodic revolution of a spot. Why 
do not the spots move in straight lines? Show how they curve. Tell 
what you can about the irregular movements of the spots. 

47. Illustrate how suddenly they change. What can you say about 
their periodicity? Who discovered this? Is there any connection be- 
tween the solar spots and the aurora ? 

48. Tell about the influence of the planets on the spots. Do the 
spots affect the truitfulness of the season ? Does the temperature of the 
spots differ from that of the resc of the sun ? Are the spots depressions 
in the sun ? 

49. How much darker are they than the adjacent surface ? Is the 
sun brighter than the Drummond light? {Ans. "The sun gives out 
as much light as one hundred and forty-six lime-lights would do, if 
each were as large as the sun and were burning all over.") 

50. What are the faculse ? Describe the mottled appearance of the 
sun. What is the shape of the bright masses ? What is a granule ? 
What is its size ? 

51. 52. Describe the constitution of the sun according to Wilson's 
theory. How are the spots produced ? The faculae ? The penumbra ? 
The nucleus ? The umbra ? 

53. 54- What is the present theory (" KirchhofFs Theory ") ? Name 
the four different portions of the sun. Define the nucleus. The photo- 
sphere. The chromosphere. The corona. What are the protuber- 
ances? How are the spots produced ? The umbra? The penumbra? 
Upon what discoveries does this theory depend (p. 262)? What is the 
cause of the heat of the sun ? Will the heat ever cease ?* 

The Planets. — Name the six characteristics common to all the 
planets. Compare the two groups of the major planets. 

57, 58. Draw an ellipse, and name the various parts. Define the 
ecliptic. The plane of the ecliptic. Why is the ecliptic so called ? 
Define the ascending node. The descending node. Line of the 
nodes. Longitude of the node. Tell what you can with regard to 
the comparative size of the planets. 

* If we accept the Nebular hypothesis (p. 256), we must suppose that the heat is 
produced by the condensation of the nebulous matter and consequent chemical 
changes. The sun is radiating its heat constantly, and, at some time, its light will go 
out, in turn, as that of the earth and the planets has before it. This theory is of especial 
interest, as it shows that the sun, as well as the solar system, has a certain fixed exist- 
ence, and that, " like all natural objects, it passes through its regular stages of birth, 
vigor, decay, and death, in one order of progress."— iV>2£>£j>//7;5. 



THE SOLAR SYSTEM. 297 

60. What is a conjunction ? Name the earliest that are recorded. 

61-3. What do you say concerning the probability of the planets being 
inhabited ? :f State the conditions of life on the different planets. What 
are the two divisions of the planets? 

64. What causes the apparently irregular movements of the planets ? 
Define heliocentric and geocentric places. Illustrate. In what part 
of the sky is an inferior planet always seen? Define inferior and 
superior conjunction. Greatest elongation. 

65. Why is a star atone time " evening " and, at another, " morning 
star " ? What is a transit ? 

65, 66. Explain the retrograde motion of an inferior planet. (This 
motion, it will be remembered, was one that sorely puzzled the 
ancients.) Describe the phases of an inferior planet. 

67. Why does an inferior planet have phases ? Define gibbous. 
Explain the opposition and conjunction of a superior planet. 

63. Explain its retrograde motion. Must a superior planet always 
be seen in the same part of the sky as the sun? Define quadrature. 
Can au inferior planet be in quadrature ? 

69, 70. Which retrogrades more, a near or a distant planet? Define 
a sidereal and a synodic revolution of an inferior and a superior planet, 
and tell what you can about each. In what case would there be no 
difference between a sidereal and a synodic revolution ? Why is a planet 
invisible when in conjunction? When is a planet evening, and when 
morning star ? 

71. Tell what you can about the supposed discovery of a planet in- 
terior to Mercury. What name and sign have been given to it? 

Mercury. — Definition and sign ? Describe the appearance of Mer- 
cury, and where seen. What was the opinion of the ancients ? Of the 
astrologists ? Of chemists ? Why is it difficult to see this planet ? 
When can we see it best ? 

73. What is the peculiarity of its orbit? What is Mercury's greatest 
elongation from the sun? Why does this vary? What is Mercury's 
distance from the sun? What is its velocity? What is the length of 

* The uniformity of Nature is a most effective argument in this direction. Light 
travels everywhere through the universe at the same rate. The elements of star, 
planet, and the earth are the same. The sun, which may be considered as the mother 
of the earth, is composed of the same materials. The laws of gravitation rule so 
absolutely that the satellite of Sirius was not discovered until after it was observed 
that an unknown influence afiected the star. " The uniformity oflaw and matter is 
proof that there must be through the universe organizations similar to those of our 
system. We see the result of these laws in the world we inhabit, and we cannot 
doubt that the same powers and the same materials have produced organizations 
similar to these of the earth in millions of other places, though we can only 
philosophically suppose their existence, not practically prove it." — W. Meyer, 



298 QUESTIONS FOR CLASS USE. 

its day? Of its year ? What is the difference between its sidereal and 
synodic revolutions ? What is its distance from the earth? 

74-6. Show why its greatest and least distances vary so much. What 
is its diameter? Volume? Density? Force of gravity ? Specific 
gravity? How much would you weigh on Mercury ? (Mercury's force 
of gravity as compared with that of the earth is .46.) Describe its sea- 
sons. (If the pupil does not understand pretty well the subject of the 
terrestrial seasons, it would be well here to read carefully page 95, 
et seq.) What is the temperature? The appearance of the sun ? Has 
Mercury any moon? What is the appearance of the planet through a 
telescope? What do these phases prove? What do we know of the 
mountains and valleys upon Mercury? The atmosphere? Have we 
any recent observations ? 

77. Venus.* — Definition and sign? Ancient names? Appearance 
to us? When brightest? Can Venus be seen by day? Illustrate. 

78. Describe the orbit of Venus. What is the distance of Venus 
from the sun? Velocity? Length of the year? Day? Difference 
between the sidereal and synodic revolutions? Distance from the 
e'arth? 

79. How near may Venus approach us? How does her apparent 
size vary ? When is Venus the brightest ? What is her diameter ? 
Volume? Density? 

80. Force of gravity ? (The force of gravity on the surface of Venus 
is .82 that of the earth.) Does the force of gravity increase or decrease 
with the mass or volume of the body ? Describe the seasons upon 
Venus. 

81. 82. Describe the telescopic appearance of Venus. Who dis- 
covered the phases of Venus ? What was the effect of this discovery? 
What proof have we that Venus possesses a dense atmosphere ? Has 
Venus a moon? 

83. Earth. — Sign ? What is the appearance of the earth from the 
other planets? Do we, then, live on a star? Is it probable that the 
earth was always dark and dull as it now seems to us?* How does 

* Venus is the only planet mentioned by Homer — 

Oio? S'aarrjp elcri ixer" aa-Tpacri vu/ctos ajaoAyw 
*E<T;repos 6s KaAAiaros ev Ovpavw icrraTai acmjp 

Iliad, xxii. 317. 

* Probably not. The earth was doubtless once a glowing star, like the sun. Its 
crust is only the ashes and cinders of that fearful conflagration. The rocks are all 
burnt bodies. The atmosphere is only the gas left over after the fuel was all con- 
sumed. Every organic object has been rescued by plants and the sunbeam from the 
grasp of oxygen. 



THE SOLAU SYSTEM. 299 

the size of the earth compare with that of the other planets? What is 
the shape of the earth ? What is its exact diameter ? (See Table in 
the Appendix.) 

84. Circumference? Density? Weight? What comparison may be 
made to illustrate its inequalities? How do you prove the rotundity 
of the earth ? ~ 

85. Why can we see further from the top of a hill than from its base? 
Why is the horizon a circle ? 

86-7. Give some illustrations of apparent motion. Is it, then, natu- ' 
ral for us to transfer motion ? Under what conditions do you think 
this occurs? Explain the cause of the rising and setting of the sun 
and stars. Who first explained these phenomena in this manner? 
What do you say of its simplicity ? 

88. What is the cause of day and night? Do all places on the 
earth revolve with equal velocity ? Illustrate. At what rate do we 
move? Why do we not perceive our motion? 

89. What would be the effect if the earth were to stop its rotation ? 
Is there any danger of this catastrophe? How is the length of the day 
increasing? Is the amount appreciable? 

90-r. Draw the figure, and show how the stars move daily through 
unequal orbits and with unequal velocities. Describe the appearance 
of the stars at the N. Pole. At the Equator. At the S. Pole. 

92—3. Describe the path of the earth about the sun. Define eccentri- 
city. What is the amount of the eccentricity of the earth's orbit? Is 
this stable ? Do we see the same stars at different seasons of the year ? 
Why not? If we should watch from 6 p. m. t'j 6 A. 11., what portion of 
the sphere would we sec? 

94. What do we mean by the yearly motion of the sun among the 
stars? How can we see it ? What is the cause ? What is the ecliptic ? 
Why so called ? What are the equinoxes? What do you understand 
when you see in the almanac the statement that "The earth is in 
Aries?" " The sun is in Sagittarius?" etc. How many apparent 
motions has the sun ? Name them, and give the cause and effects of 
each. Has the sun any real motions? 

95. Describe the apparent motion of the sun, N. and S. How is it 
that the sun in summer shines on the north side of some houses both. 
at rising and setting, but in winter never does ? Define the obliquity 
of the ecliptic. The parallelism of the earth's axis. What do you say 
of its permanence ? Why will a top stand while spinning, but will fall 
as soon as it ceases ? 

0,7. Show bow the rays of the sun strike the various parts of the 

* It is said that aeronauts, at a proper height, can distinctly see the curving form 
of the earth's surface. 



300 QUESTIONS FOR CLASS USE. 

earth at different angles at the same time. Show how the angles vary 
at different times. Is the sun really hotter in summer than in winter? 
Why does it seem to be? Why is it warmer in summer than in win- 
ter? What effect upon the temperature has the difference in the length 
of the summer and the winter day? 

98-100. Explain the cause of equal day and night at the equinoxes. 
Why are our days and nights of unequal length at all other times ? 
Why does the length vary at different seasons of the year ? How do 
the seasons, &c, in the N. Temperate Zone compare with those in the 
S. Temperate Zone ? Describe the yearly path of the earth about the 
sun — (1), at the summer solstice ; (2), at the autumnal equinox ; (3), at 
the winter solstice ; (4), at the vernal equinox ; (5), the yearly path 
finished back to the starting-point. Is the division of the earth's 
surface into zones an artificial or a natural distinction? Who in- 
vented it? 

101. How much nearer are we to the sun in winter than in summer? 
Why is it not warmer in winter? How is it in the South Temperate 
Zone ? When do the extremes of heat and cold occur? Why do they 
not occur exactly at the solstices? 

102. Why is summer longer than winter ? Does the earth move 
with the same velocity in all parts of its orbit? Describe the curious 
appearance of the sun at the North Pole. In Greenland, at what part 
of the year will the midnight sun be seen due north ? What is the 
length of the days and nights at the Equator ? 

103. Describe the results if the axis of the earth were perpendicular 
to the ecliptic. If the equator were perpendicular to the ecliptic. 

104-5. Define the precession uf the equinoxes. W r ho discovered 
this fact ? At what rate does this movement proceed ? What time will 
be required for the equinoxes to make an entire revolution? What 
are the results of precession ? What star was formerly the Polar Star ? 
(See p. 219.) 

106-9. Explain the cause of precession. How does the spinning of a 
top illustrate this subject ? In what way is the force which acts on a 
spinning-top opposite to that which produces precession ? What is 
Nutation? What is the cause of the nodding motion? How does the 
moon's influence compare with that of the sun ? What is the effect of 
Nutation ? 

no. What is the real path of the N. Pole through the heavens? Is 
the obliquity of the ecliptic invariable? What is the period of this 
oscillation ? 

in. What causes combine to produce this nodding motion we have 
described? Why are the tropics located where they are? Is their 
position on the earth permanent? What effect does precession have 



THE SOLAR SYSTEM. 301 

on the latitude of the stars ? What is meant by the line of apsides of 
the earth's orbit? Is this permanent ?* What is the Great Year of 
the Astronomers ? 

112. At what season of the year is the earth now in perihelion ? 
When was it in perihelion in the autumn? When in the winter? 
When will perihelion occur in the spring? When in summer? 
1 When will the cycle be completed ? What provision is there for per- 
manence in the midst of these changes? 

113-14. What is refraction? Its effect? Upon what principle of 
Optics is this based? How does refraction vary ? Are the sun and 
moon ever where they seem to be ? Is the real day longer or shorter 
than the apparent one ? 

115. Describe the apparent deformation of the sun and moon near 
the horizon. Explain. Why are not these bodies apparently deformed 
in the same way when they are high in the heavens ? Why do they 
appear smaller in the latter case ? (See Fig. 48, p. 124.) What causes 
the hazy appearance ot the heavenly bodies near the horizon ? 

116. What is the cause of twilight ? How long does it last? Is it 
the same at all seasons of the year ? In all parts of the earth ? 

117. Where is it the longest? Shortest? State the cause of this 
variation. What is diffused light ? What would be the effect if the 
atmosphere did not act in this way? Is there really any sky in the 
heavens? What is the cause of the appearance? What is aberration 
of light ? 

118. Illustrate this phenomenon. State two reasons why we never 
see the sun where it really is. 

119. What is the general effect of aberration ? Define parallax. Il- 
lustrate. 

120-21. Define true and apparent place. How does parallax vary? 
What is the practical importance of this subject (pp. 36, 278)? Define 
horizontal parallax. What is the sun's horizontal parallax? What is 
the annual parallax? 

The Moon. — Signs ? Describe the moon's orbit. What is the 
moon's distance from the earth ? Illustrate. What is the difference 
between her sidereal and synodic revolutions? What is the real path 
of the moon? (Imagine a pencil fastened to the spoke of a wheel, and 
the wheel rolled by the side of a wall on which the pencil is constantly 
marking.) How often does the moon turn on her axis ? What is the 

* M The line of equinoxes of the earth's orbit, as we have seen, has a slow left- 
handed retrograde i7totion of 5o".2 each year, called the precession of the equinoxes ; 
and the line of apsides has a still slower right-handed direct motion of n".2a ; and 
in consequence of the motion of both chese lines, the angle formed by them changes 
through 6i".49 each year, so as to complete an entire revolution in 21,077 years." 



302 QUESTIONS FOR CLASS USE. 

moon's diameter? Volume? How does her apparent size vary? 
Why does she appear larger than she really is ? 

124. Why does the crescent moon appear larger than the dark body 
of the moon? When ought the moon to appear the largest? Do all 
persons think the moon to be of the same apparent size? 

125. Explain the three librations of the moon. How does moonlight 
compare with sunlight? Is there any heat in moonlight? 

126. Does the center of gravity in the moon coincide with that of 
magnitude ? Has the moon any atmosphere ? What proof have we of 
this ? (Ans. (1). We see but slight, if any, appearance of twilight on 
the moon. (2). When the moon passes between us and a star, it does 
not refract the light of a star, so that the atmosphere cannot be suffi- 
cient to support more than T ^ of an inch of the mercurial column.) 
What must be the effect of this lack upon the temperature of the moon's 
surface ? State Langley's observations upon Mount Whitney. How 
does the earth appear from the moon? 

127-9. What is the earth-shine ? How is it caused ? What is it called 
in England ? Describe the path of the moon around the earth, and the con- 
sequent phases. Why is new moon seen in the west and full moon in the 
east ? Why can we sometimes see the moon in the west after the sun rises, 
and in the east before the sun sets ? What is the length of a lunar 
month?* What do" we mean by the moon's running high or low? 
What is the cause of this variation ? Is it of any use ? 

130-1. What is harvest moon ? What is the cause ?f 

132. Explain the cause of "Dry Moon" and "Wet Moon." What 
are nodes? How much is the moon's orbit inclined to the ecliptic — 
our ideal sea-level? What is an occultation? What use does it sub- 
serve? Describe the seasons, heat, etc., on the moon. 

135-7. Describe the telescopic appearance of the moon. Are the 
mountains the light or the dark portions? What can you say about 
them? The gray plains ? The rills? The craters? What are the pecu- 
liar features of the lunar landscapes ? Are the lunar volcanoes extinct ? 

* " The moon's sidereal period is not constant, and a compaiison of modern with 
ancient observations shows that it has undergone an acceleration since the period of 
the Chaldean observations of eclipses made 720 b. c. Several explanations have been 
given by Laplace and others, of the supposed cause of the acceleration of the moon's 
mean motion ; but it is highly probable that it is a pseudo-phenomenon* that owes its 
origin to a real lengthening of the time of rotation of the earth (which is the unit 
of astronomical time), caused by the friction of the sea and atmosphere." 

+ It will aid in understanding the cause of harvest moon, if one gets clearly in 
mind the fact that the moon when full is always in the opposite part of the heavens 
from the sun. At the time of the autumnal equinox, i. c. when the sun is at the 
autumnal equinox, (or in Libra, note, p. 94.) the moon must be at the vernal equinox, 
(or in Aries.) The least retardation of the moon, which occurs at this time, happens, 
therefore, in September. 



SOLAR SYSTEM. 303 

138. Eclipses. — Vv T hcn can an eclipse of the sun occur? Show how 
a solar eclipse may be total, partial, or annular. Define umbra. Pe- 
numbra. Central eclipse. State the general principles of a solar 
eclipse. What curious phenomena attend a total eclipse?* What 
are Baily's Beads ? What is the corona ? Describe the effect of a total 
eclipse. What curious custom prevails among the Hindoos? What 
is the Saros? Is it now of any value? What is the metonic cycle? 
Explain its use. What is the golden number? What is the cause of a 
lunar eclipse ? Why are lunar eclipses seen oftener than solar ones ? 
How were total eclipses formerly regarded? 

147. The Tides, f — Define ebb. Flow. How often does the tide 
happen ? Explain the cause. Why does the tide occur about fifty 
minutes later each day? Why is there a tide on the side opposite the 
moon? The sun is much larger than the moon ; why does it not pro 
duce the larger tide ? W 7 hat effect has the friction of the tides produced 
upon the earth? What theory upon this topic has Professor Ball 
advanced ? What is meant by the differential effect of the moon ? Why 
is not the tide felt out at sea? What is spring-tide ? Neap-tide? Why 
does the tide differ so much in various localities? Tell about the 
height of the tides at different points. Why is there no tide on a lake ? 
Is the tidal wave a forward movement of the water ? 

150. Mars. — Definition and sign? Describe the appearance of this 
planet. When is it brightest? What is its distance from the sun? 
Velocity? Day? Year? Distance from the earth? What is the pecu- 
liarity of its orbit? What is the diameter of Mars? Its volume and 
density as compared with the earth? How far would a stone fall on 
its surface the first second ? Who discovered its moons ? What is the 
peculiarity of these tiny globes? What are the peculiar telescopic 
features of Mars? What is the cause of its ruddy color? What are 
the snow-zones ? Can we watch the change of its seasons ? 

* Lockyer, describing the beginning of a total eclipse, says : " One seems in a new 
world — a world filled with awful sights and strange forebodings, and in which still- 
ness and sadness reign supreme : the voice of man and the cries of animals are hushed ; 
the clouds are full of threatenings and put on unearthly hues ; dusky, livid, or purple, 
or yellowish crimson tones chase each other over the sk5^ irrespective of the clouds. 
The very sea is responsive and turns lurid red. All at once the moon's shadow comes 
sweeping over air, and earth, and sky, with frightful speed. Men look at each other 
and behold, as it were, corpses, and the sun's light is lost." — Gillis. in his observa- 
tions upon the eclipse of 1859, as witnessed by him in Peru, remarks: "At 1.54, the 
moment of totality, the attendants, catching sight of the corona, dropped on their 
knees, and shouted, " La Gloria ! La Gloria ! " 

t As the tidal wave does not move as rapidly as the earth does, the water has an 
apparent backward motion. It has been suggested that this (as well as the friction 
of the atmosphere) acts as a break on the earth's diurnal revolution. It has been 
shown that the moon's true place can be best calculated if we suppo-re that the side- 
real day is shortening at the rate of i^H^ of a second in 2,400 years. (See page 8g.) 



304 QUESTIONS FOR CLASS USE. 

154. Minor Planets (Asteroids).*— Give Bode's law. Tell how the 
first of these planets was discovered. How many are now known? 
Are they probably all discovered ? Describe some of these " pocket 
planets". Do they all lie within the Zodiac? What is their origin ? 
(Am. According to the nebular hypothesis, the ring of matter broke 
up into numberless small bodies, instead of aggregating into one large 
planet.) Give some of the names and signs. 

157. Jupiter. — Definition and sign? Describe his appearance. De- 
scribe his orbit. What is his distance from the sun? Velocity? Day? 
Year? Distance from the earth? Diameter? Volume? Density? 
Centrifugal force ? Force of gravity ? Figure? Describe his seasons. 
Upon what does the change of seasons in any planet depend? What 
must be the appearance of the Jovian sky? Describe the telescopic 
features of Jupiter. Are Jupiter's moons visible to the naked eye? 
What are their names ? What is their size ? What space do they 
occupy? Describe the eclipse of Jupiter's moons. Define immersion, 
emersion, and transit. How rapidly do the satellites revolve ? What 
can you say of the frequency of eclipses on Jupiter? Describe the belts. 
Why are they parallel to its equator? How was the velocity of light 
discovered? Does Jupiter emit light? Is it probable that a solid 
crust has formed over this planet ? In what way is Jupiter repro- 
ducing the earth's history? 

164. Saturn. — Definition and sign ? Describe Saturn's appearance 
in the heavens. How rapidly does this planet move through the sky? 
What is its distance from the sun ? What is the peculiarity of its 
orbit ? What is its velocity ? Year ? Day ? Distance from the earth ? 
Diameter? Volume? Density? Force of gravity ? Describe its sea- 
sons. Has it any atmosphere ? Who discovered the rings of Saturn ? 
Describe them. Which are the Bright Rings ? Which is the Dusky 
Ring? Are they stationary? Explain their phases. Of what are 
they composed? Does Saturn emit light? Describe Saturn's belts. 
Describe Saturn's moons. The scenery on Saturn. 

170. Uranus. — Definition and sign ? How was this planet dis- 
covered ? Tell of its previous observation by Le Monier. Is Uranus 
visible to the naked eye ? What is its distance from the sun ? Year ? 

* " It may surprise some persons to learn that the total mass of the two or three 
hundred small planets which have been discovered between the orbits of Mars and 
Jupiter, is sufficient only to make a globe a little over 400 miles in diameter. In other 
words if our globe were divided into 8,000 equal parts, one of these parts would equal 
in bulk and in weight the total of all these asteroids. Or, cut the earth through the 
equator, then take a section of about three-fourths of a mile in thickness, and it would 
furnish material for all these small planets and something remaining. It would seem 
that the solar system could not be much damaged, if some of these small planets 
should drop out of their courses and join some of the larger ones." 



THE SOLAR SYSTEM. 305 

Diameter? Density? Describe its seasons. Telescopic features. 
Satellites. Pecul'arity of its moons. 

172. Neptune. — Definition and sign? What is the appearance of 
this planet in the sky ? Give an account of its wonderful discovery. 
What is its distance from the sun? Year? Velocity? Diameter? 
Volume? Density? Do we know anything of its seasons? Why not ? 
What is the appearance of the heavens? What are the telescopic 
features of Neptune? Has Neptune any moon? What advantage 
have the Neptunian astronomers ? 

175. Meteors, Aerolites, and Shooting-Stars. — Define an aero- 
lite. A shooting-star. A meteor. Give some account of the fall of 
aerolites. What elements are found in aerolites? How can an aero- 
lite be distinguished? Give an account ot wonderful meteors. Of 
shooting-stars. 

176. Describe the showers of 1799 and 1833.* The shower of 1866. At 
what intervals did these showers occur? Why was not the shower of 
1866 seen in this country ? (Aus. Our side of the earth was not turned 
toward the meteors.) What is the average number of meteors and 
shooting stars daily? Why do we not see more of them? In what 
months are they most abundant ?f Describe the origin of meteors and 
shooting-stars. What is their velocity ? What causes the light ? The 
explosion often heard ? What is the theory of meteoric rings ? What 
is their shape? How do these streams of meteoroids account for the 
showers at regular intervals? What is the period of the November 
ring? Why is the August shower so uniform, while the November 
one is periodic?:}: What is the relation between meteors and comets? 

* A southern planter, describing the effect of the star-shower of 1833, says : " I 
Avas suddenly- awakened by the most distressing cries that ever fell on my ears. 
Shrieks of horror and cries for mercy I could hear from most of the negroes of three 
plantations, amounting in all to about Coo or 800. While earnestly listening for the 
cause, I heard a faint voice near my door calling my name. I arose, and taking my 
sword, stood at the door. At this moment I heard the same voice still beseeching mc 
to rise, and saying, ' Oh, my God, the world is on fire !' I then opened the door, 
and it is difficult to say which excited me most, the awfulness of the scene or the 
cries of the distressed negroes. Upwards of one hundred lay prostrate on the ground, 
some speechless, and some with the Litterest cries, with their hands raised, implor- 
ing God to save the world and them. The scene was truly awful for never did rain 
fall much thicker than the meteors towards the ear.h : cast, west, north, and south, it 
was the same." 

t It has been noticed, from very early times, that the night of the 10th cf August 
. (St. Laurence's Day) is especially favorable for the occurrence of shooting-stars; and 
in Catholic Ireland, these stars, on the Toth of August, are always called the " tears 
of St. Laurence the Martyr," who was put to death by being broiled upon a gridiron. 

% The fact that the November meteoroids are collected in a shoal instead of be- 
ing distributed uniformly through the orbit gives color to the. idea that this stream 
has. not been long a member of the solar .system. ";I.n. 1.867, Levcrrier stated-his.be- 



306 QUESTIONS FOR CLASS USE. 

What do you mean by the radiant point? What is the height of 
meteors ? Weight ? 

185. Comets. — How were comets looked upon by the ancients? Il- 
lustrate. Define the term comet. What is the tail ? s Ihe nucleus? 

lief that the November meteors form the remains of some comet that had been re- 
cently introduced into the solar system by the attraction of one of the large outer 
planets. He found that the year a. d. 126 would give a position to the planet Uranus 
capable of producing such an effect, by converting the parabolic path of a comet into 
the path now described by the November meteors. In the year a. d. 137, the changed 
path of the comet for the first time came near the earth in her orbit round the Sun, 
since which year the petrified comet or shower of stones has completed 52 entire re- 
volutions, the last of which terminated on the 13th of November, 1866. Theophanes 
of Byzantium relates that in November, a. d. 472, the sky at Constantinople appeared 
to be on fire with flying meteors. This corresponded with the tenth revolution of 
the November meteors. — Conde, in his history of the dominion of the Arabs, speak- 
ing of the year a. d. 902, states that in the month of October (13th), en the night of 
the death of King Ibrahim Ben Ahmed, an immense number of falling stars were 
seen to spread themselves over the face of the sky like rain, and that the year in ques- 
tion was thenceforth called the ' Year of Stars.' This year corresponded to the 
twenty-third revolution of the November meteors. — A similar shower of stars took 
place on the 17th of October, a. d. 934. — On the 14th of October, a. d. 1002, a remark- 
able shower of shooting-stars is noted by the Arab astronomers and historians, cor- 
responding with the completion of the twenty-sixth revolution of the November 
meteors.— It is related in the annals of Cairo that on the 19th of October, a. d. 1202, 
the stars appeared like waves upon the sky, towards the east and west ; they flew 
about like grasshoppers, and were dispersed from left to right. This shower corres- 
ponded with the thirty-second revolution of the November meteors.— On the 22nd of 
October, a. d 1366, a shower of stars was noted, corresponding with the thirty- 
seventh revolution of the November meteors. — A similar phenomenon (forty-second 
revolution) was observed on the 25th of October, A. d. 1533. — The forty-seventh 
revolution was noted on the 9th of November, a. d. 1698. — The fiftieth revolution, 
observed by Humboldt and Boupland, on the 12th of November, a. d. 1799, as already 
remarked, first led modern astronomers to speculate on the true nature of these re- 
markable periodic phenomena. — The early observations of this meteoric shower were 
dated on the 12th of October, and during 52 revolutions the intersection of its orbit 
with that of the earth has moved on to the 14th of November — Mr. Adams has shown 
this movement of nodes to be a consequence of the attractions of the superior planets^ 
and has finally demonstrated the truth of the cometary origin of the November 
meteors.'' — Houghton. 

* " Comets are almost always accompanied by tails, which are placed in the line 
joining the Sun and Comet, and on the side opposite to the Sun. Exceptions to this 
rule, though rare, sometimes occur. For example, the tail cf the Comet of 1577 de- 
viated 21 from the line joining the Sun and the Comet, and the tail of the Comet of 
1680 diverged 5 from the same line. Comets have been occasionally observed with 
two tails, one in the usual position, and the other in nearly an opposite direction, or 
towards the Sun. The angle between the two tails, when such a phenomenon has 
been observed, has always been very considerable, varying from 140 to 170 . 
This rare phenomenon of two tails is supposed to be connected with certain rapid 
changes which the gaseous substance of the Comet is observed to undergo on ap- 
proaching the Sun. There are many instances on record, in which thetails of Comets 
-were, observed to stretch through 100 ° of the celestial sphere, and. the apparent 



THE SIDEREAL SYSTEM. 307 

The head? The coma? Does each comet necessarily possess all 
these parts ? How would a mere round, fleecy mass be known to be 
a comet ? What mistake did Herschel make in looking, as he sup- 
posed, at one of this kind (p. 171)? Where do comets appear? In 
what direction do they move? How does a comet look when first 
seen ? Describe the approach of a comet to the sun. Upon what does 
the time of greatest brillianc}' depend ? What do you say of the num- 
ber of the comets ? What was Kepler's remark ? Why do we not see 
themoftener? Where did Lockyer see one? Describe the orbits of 
comets. Which class has been calculated ? Which classes never re- 
turn? Describe the difficulty of calculating a comet's orbit. Name 
the periods of some comets. What has been the distance from the sun 
of some noted comets ? Velocity ? What do you say of the density of a 
comet? Illustrate. Is there any danger of our running against a 
comet ? Do comets shine by their own or by reflected light ? Tell what 
you can of their variation in form and dimensions. Give some account 
of the comets of 1811. 1835, ar) d 1843. For what is Biela's comet 
noted ? (Ans. " A very remarkable phenomenon attended the perihelion 
passage of this comet in the latter end of 1845. It became divided 
into two comets, which did not again re-unite, but traveled along to- 
gether in similar orbits. This unique phenomenon was noticed for 
the first time in America on the 29th of December. The greatest dis- 
tance observed between these two fragments of Biela's comet, before 
their final disappearance, was about two-thirds of the moon's distance 
from the earth.") For what is Encke's comet noted ? What is its period ? 
Give some description of Donati's comet. The comet of 1882. 

196. Zodiacal Light. — Where can this be seen ? What is its appear- 
ance? Where is it brightest? What is its origin ? 

III. THE SIDEREAL SYSTEM. 

203. Tell something of the appearance of the heavens at Neptune's 
distance from the sun — our starting-point. Do we ever see the stars? 
What do we see, then ? Which star is nearest the earth ? What is its 
parallax? Its distance? How long would it take light to reach the 
nearest star? How would the earth's orbit appear at that distance ? Our 
sun? How long does it take for the light of the smaller stars to reach 
the earth ? What can you say of the motion of the fixed stars? Illustrate. 

length of the tail is known to undergo most rapid changes. We shall mention on'.y 
one case as an example of this phenomenon. The Comet of 1618 presented to the 
Danish astronomer, Longomontanus, a tail of 104 ° in length, while it had been 
measured by Kepler a few days previous, and ascertained to be only 70° long." 



308 QUESTIONS FOR CLASS USE. 

What proof have we that the fixed stars are suns ?* Describe the motion 
of the solar system. Is the center known ? How many stars can 
we see with the naked eye ? With a telescope ? Have all the stars 
been discovered ? What is the cause of the twinkling of the stars? 
Do we know anything of the magnitude of the stars ? Name the points 
of difference between a planet and a fixed star. What do you mean 
by a star of the first magnitude ? How many are there ? Of the second 
magnitude ? How many sizes can one see with the naked eye ? What is 
the cause of the difference in the brightness ? How are the stars named ? 
Describe the division of the stars into constellations. Is there any real 
likeness to the mythological figures ? Name any figure which seems 
to you well founded. Are the boundaries distinct ? Who invented 
the system ? State the possible meaning of the signs of the Zodiac and 
their origin. Explain why the signs and the constellations of the 
Zodiac do not agree. What causes the appearance of the constella- 
tions? Would they appear as they now do, if we should go out into 
space among them ? Are the present forms permanent ? State the value 
of the stars in practical life. What were the views of the ancients with 
regard to the stars? Describe the division of the stars into three zones. 
214. The Constellations. — The questions on these are uniform : 
(1) description, (2) principal stars, and (3) mythological history. Therefore, 
they need not be repeated with each constellation. What are the 
pointers? Does Polaris mark the exact position of the North Pole? 
How many times per day is Polaris on the meridian of any place? 
Explain how this applies in navigation or surveying. State how the 
amount of the variation from the true north will change through the 
ages. What star will ultimately become the pole-star ? What curious 
facts are stated concerning the Pyramids ? What do you say of the dis- 
tance of Polaris ? How may latitude be calculated' by means of Polaris ? 
What stars never set in our sky? What stars never rise?f Will the 

* Sinus shines at least 200 times as brightly as our sun would shine if set beside 
it. Assuming its surface to be equally brilliant, this would imply, in comparison 
with our sun, a diameter 14 times and a volume 3,000 times as great. Its luster, 
however, seems higher than the sun's, but, even making allowance for that, we must 
still consider this giant sun to be at least 1,000 times as large as our own orb. Re- 
cent evidence tends to show that its rate of recession from us is diminishing, so that 
we may expect this to change into a motion of approach. Here is a hint that Sirius 
is travelling in a mighty orbit with movements carrying it alternately from and toward 
us. — Proctor. 

t All stars whose north polar distance is less than the latitude of any p'acc, will 
never set at that place, and all stars whose south polar distance is less than the latitude, 
will never rise. The Greeks and the Romans were familiar with the fact that cer- 
tain stars never descend below the horizon. The following quotations are interesting : 
"Immunemque cequoris Arcton." 

Ovid, Metam. xiii. 293. 
..- • .-- •--■ .. . „..■ .<: - «• Arctos -■■•■•■ ...--.--'..-■ ----- 
j&quoris expertes." Id. 726—7. 



THE SIDEREAL SYSTEM. 309 

Big Dipper always appear as now ? Name three bright stars near the 
first meridian. {Arts. " Andromedae, y Pegasi, and S Cassiopeiae.) How 
many degrees of longitude correspond to an hour of time ? At what 
rate is Sirius receding from the earth? How has this motion been dis- 
covered? (See page 261.) 

239. Double Stars, etc. — Does any star appear double to the naked 
eye ? How many have been found by the use of the telescope ? What 
is an optical double star? Are all double stars of this class ? Describe 
the revolution of a binary system. What other combinations have been 
discovered? What are their periods? Orbits? Mass? Are these 
compauion stars as close to each other as they seem ? 

241. Name some prominent colored stars. Do their colors ever 
change ? Which colors would indicate the hottest star ? What is the 
probable effect in a system having colored suns? 

242. What are variable stars ? Describe the changes of Algol. Of 
Mira. What is the cause ? 

243. What are temporary stars? Describe the one seen in Cassio- 
peia. The one in Corona Borealis, in 1866? What are lost stars? 
Can you give any explanation of this phenomenon? Of what did the 
star of 1866 consist ? Are these stars destroyed ? Is the process of 
creation now complete ? 

245. What are star clusters ? Name several. Is such a grouping a 
mere optical effect ? Are they probably as closely compacted as they 
seem to be ? 

246. What are nebulas ? How do they differ from clusters ? Is it 
probable that all nebulae will be resolved into clusters ? What is the 
general belief concerning nebulae ? What has spectrum analysis proved 
some of the nebulas to be ? Where are they most abundant? What 
can you say about their distances? Into how many classes, for con- 
venience, are they divided ? Describe and illustrate the elliptic nebu- 
las. What is said of the distance of the great nebula in Andromeda ? 

' 'ApKTOv 6' riv Koi afxa^av ejriicA.Tjcrti' Kakiovaiv, 
'Ht' avrov arpecfreTai, ko.1 t 'Opitora fioKevet, 
Git\ S' appopo? €<ttc Koerpmv 'Qiceavoio, 

Iliad, xviii. 487 — 9, and Odys. v. 273—5. 
*Ap/croi Kvaveov ire^uAaypeVoi 'niceavoLO. 

Aratus, Ph/BNOm. 48. 
" Arctos oceani metuentes sequore tingui." 

Virg. Georg. L 246. 
In order to understand the meaning of the expressions we^vkaytMeuoL 'Sl/ceavolo, and 
'• cequoris expertes" as used by a Greek or Italian, we should remember that the 
north polar distance of 17 Ursae Majoris is 39 56' 48" ; and since the latitude of Athens 
is 37 58', and that of Naples 40° 50', an inhabitant of the former city would see this 
star descend below the northern horizon for a small portion of its course • and an 
inhabitant of Naples would see it sink within 3' of the horizon, so as just to move 
along its northern edge. 



310 QUESTIONS FOR CLASS USE. 

The number of stars it contains ? Describe the annular nebula;. What 
is said of the " ring universe " in Lyra ? Its diameter ? Describe the 
spiral nebula in Canes Venatici. Describe the planetary nebulae. 
What is said of the number and size of these "island universes"? 
Describe the fantastic appearance of the irregular nebulae. 

251. What are nebulous stars? What is their structure? What 
are variable nebulas ? Give instances. What is said of double 
nebulas ? Is anything definite known with regard to them ? What 
are the Magellanic clouds ? 

253. Describe the Milky-way. What can you say of the number of 
stars in the Galaxy? Are the stars uniformly distributed ? 

254. What is Herschel's theory of the constitution of the universe ? 
If this theory be true, what is our sun ? 

255. Give an account of the Nebular hypothesis. What is said of 
Saturn's rings ? May they ultimately disappear ? 

259. What is spectrum analysis? Name the three kinds of spectra. 
What colored rays will a flame absorb?* Describe the spectroscope. 
What are Fraunhofer's lines? What is known of the constitution 
of the sun? What proof have we that iron exists in the sun? What 
elements have been found in the sun ? What proof have we that the 



* The power which gases possess of cutting' out the particular lines which belong 
to the color that each emits has been beautifully illustrated by Prof. Newcomb. He 
says: "• Suppose nature should loan us an immense collection of many millions of 
gold pieces, out of which we were to select those which would serve us for money, 
and ieturn her the remainder. The English rummage through the pile, and pick out 
all the pieces which are of the proper weight for sovereigns and half-sovereigns ; the 
French pick out those which will make five, ten, twenty, or fifty-franc pieces; the 
Americans the one, five, ten and twenty dollar pieces, and so on. After all the suit- 
able pieces are thus selected, let the remaining mass be spread out on the ground 
according to the respective weights of the pieces, the smallest pieces being placed in 
a row, the next in weight in an adjoining row, and so on. We shall then find a num- 
ber of rows missing: one which the French have taken out for five-franc pieces; 
close to it another which the Americans have taken for dollars ; afterwards a row 
which have gone for half-sovereigns, and so on. By thus arranging the pieces, one 
would be able to tell what nations had culled over the pile, if he only knew of what 
weight each one made its coins. The gaps in the places where the sovereigns and 
half-sovereigns belonged would indicate the English, that in the dollars and eagles 
the Americans, and so on. If, now, we reflect how utterly hopeless it would appear, 
from the mere examination of the miscellaneous pile of pieces which had been left, to 
ascertain what people had been selecting coins from it, and how easy the problem 
would appear when once some genius should make the proposed arrangement of the 
pieces in rows, we shall see in what the fundamental idea of spectrum analysis con- 
sists. The formation of the spectrum is the separation and arrangement of the light 
which comes from an object on the same system by which we have supposed the 
gold pieces to be arranged. The gaps we see in the spectrum tell the tale of the at- 
mosphere through which the light has passed us; in the case of the coins they would 
tell what nations had sorted over the pile." — N'c-j.'comb'' s AstroitOmy, p. 228. 



THE SIDEREAL SYSTEM. 311 

stars are suns? What can you say of the similarity existing between 
the stars and our earth ? What has been discovered with regard to 
the constitution of the Nebulae? Of their relative brightness? How 
has the proper motion of the stars been shown? 

263. Time. — What two methods of measuring time? What is a si- 
dereal day? What are astronomical clocks? Tell how they are used. 
Why do astronomers use sidereal time ? What is a solar day? What 
causes the difference between a sidereal and a solar day ? To how much 
time is a degree of space equal? Which is taken as the unit, the solar 
or the sidereal day? flow long is a solar day? A sidereal day? A 
solar day equals how many sidereal hours ? A sidereal day equals how 
many solar hours ? Describe mean solar time. What is apparent 
noon? Mean noon? The equation of time? When is this greatest? 
When least? When do mean and apparent time coincide? Can a 
watch keep apparent time? How may apparent time be kept? How 
can it be changed into mean time? Tell how to erect a sun-dial. When 
will a sidereal and a mean-time clock coincide ? A mean-time clock 
and the sun-dial ? How did the ancients measure time, before the 
invention of clocks and watches ?* State the two reasons why the 
solar days are of unequal length. What is the civil day ? Who invented 
the present division ? Describe the customs of various nations. What 

* " The ancients used clepsydrse and sun-dials, to measure time. The clepsydra:, 
in its simplest form, resembled the hour-glass, water being used instead of sand, and 
the flow of time being measured by the flow of the water. After the era of Archi- 
medes, clepsydrae of the most elaborate construction were common ; but while they 
were in use, the days, both winter and summer, were divided into twelve hours from 
sunrise to sunset, and consequently the hours in winter were shorter than the hours 
in summer ; the clepsydra, therefore, was almost useless except for measuring inter- 
vals of time, unless different ones were employed at different seasons of the year. 
The sun-dial was a great improvement upon the clepsydrse ; but at night and 
in cloudy weather it could not be used, of course, and the rising, culmination, and 
setting of the various constellations were the only means available for roughly telling 
the time during the night. Indeed, Euripides, who lived 480-407 b. c, makes the 
Chorus in one of his tragedies ask the time in this form : — 

1 What is the star now passing ?' 

and the answer is : — 

1 The Pleiades show themselves in the east ; 
The Eagle soars in the summit of heaven.' 

It is also on record that as late as a. d. noSthe sacristan of the Abbey of Cluny con- 
sulted the stars when he wished to know if the time had arrived to summon the 
monks to their midnight prayers ; and in other cases, a monk remained awake, and 
to measure the lapse of time repeated certain psalms, experience having taught him 
in the day, by the aid of the sun-dial, how many psalms could be said in an hour. 
When the proper number of psalms had been said, the monks were awakened."— 
Lockyer. 



312 QUESTIONS FOR CLASS USE. 

is the origin of the names of the days?"" What is the sidereal year? 
The mean solar year? What causes the difference? What is the 
anomalistic year? How did the ancients find the length of the year? 
What error did they make ? What was the result ? Give an account 
of the Julian calendar. The Gregorian calendar. What is the mean- 
ing of the terms O. S. and N. S. ?f What country now uses O. S. ? 
When was the change adopted in England ? % How was it received ? 
How could a child be eight years old before a return of its birthday? 
When do the Jews begin their year? Why does our year begin Jan- 
uary ist ? Show how the earth is our timepiece. What influence has 
Jupiter's moons on the cotton trade? 

Celestial Measurements. — These problems are to be used 
throughout the study. They require no questions but the formal state- 
ment of the problem requiring solution. 

* It is said that the Egyptians named the seven days from the seven celestial 
bodies then known. The order was continued by the Romans. Tuesday^hey called 
Dies Martis ; Wednesday, Dies Mercurii ; Thursday, Dies Jovis ; Friday, Dies 
Veneris. In the Saxon mythology, Tius, Woden, Thor, and Friga are equivalent to 
Mars, Mercury, Jupiter, and Venus. Hence we see the origin of our English names. 

t " As an illustration of the effect of the change of style, we may instance the 
case of Washington. He was born P'ebruary n, 1732, before the change of style. 
Inasmuch as 1752 began on the 25th of March and ended on the 31st of December, he 
had no birth-day in that year ; hence, he was 20 years old on the 22nd of February, 
1753, new style. Because anniversaries are always determined according to the 
civil calendar, the birth- day of Washington is properly celebrated on the 22nd of 
February, and not on the 23d, as some have contended, on account of the day drop- 
ped in the year iSoo. 1 ' — Pec-It's Astronomy, p. 216. 

% " In England, from the 14th century till the change of style in 1752, the legal and 
the ecclesiastical year began March 25. After the change was adopted in 1752, events 
which had occurred in January, February, and before March of the old legal year, 
would, according to the new arrangement, be reckoned in the next subsequent year. 
Thus the revolution of 1688 occurred in February of that legal } r ear, or. as we should 
now say, in February, 1689 ; and it was, at one time, customary to write the date 
thus: February, i68|."— Appleton's Cyclopaedia, article on Calendar. 



GUIDE TO THE CONSTELLATIONS. 



The following is a description of the appearance of the heavens on or about the 
first day of each month in the year. 

January. (7 P. m.) — In the North, Cassiopeia and Perseus are 
above Polaris, Cepheus and Draco west, Ursa Minor is below, and Ursa 
Major below and to the east. In the East, Cancer is just rising, Canis 
Minor (Procyon) has just risen. Along the Ecliptic, Gemini is well up, 
then Taurus, Aries — reaching to the meridian, next Pisces ; Aquarius 
(letter Y) and Capricornus are just setting. In the Southeast, Orion and 
the Hare are well up. In the South, Cetus swims his huge bulk far to the 
east and west. In the Southwest* is Piscis Australis (Fomalhaut). North 
of the Ecliptic, the Triangles arc nearly in the zenith, Perseus is just 
east, below is Auriga, Andromeda lies just west of the meridian, and 
Pegasus is midway; Delphinus (the Dolphin, Job's Coffin), Aquila 
(Altair), and Lyra (Vega) are fast sinking to the western horizon ; while, 
along the Milky Way, blazes the brilliant cross of Cygnus. 

February. (7 P. m.) — In theA T orth, Ursa Major lies east of Polaris, 
Ursa Minor and Draco are below, Cepheus is west, Cassiopeia above 
and to the west. In the East, Regulus and Cor Hydrae are just rising. 
Along the Ecliptic, Leo (Regulus, the sickle) just rising, Cancer well up, 
Gemini midway, Taurus on the meridian, Aries (the scalene triangle) 
past, Pisces next, and, lastly, Aquarius just setting. In the Southeast, 
Canis Minor, Canis Major (Sirius), and Orion are conspicuous. In the 
Southwest, Cetus covers nearly the whole sky. North of the Ecliptic, 
Perseus is on the meridian, while Auriga is a little east of it ; west of 
Perseus is Andromeda, while the Great Square of Pegasus is fast 
approaching the horizon. In the Northwest, Cygnus is setting. 

Marcll. (7 p. M.)— In the North, Ursa Major lies east of Polaris, 
Draco and Ursa Minor are below, Cepheus is below and to the west, and 
Cassiopeia west. In the East, Cor Caroli is well up, toward the north- 
east, and Coma Berenices is rising. Along the Ecliptic, Leo is fully risen, 
next Cancer, Gemini reaches to the meridian, Taurus is past, Aries 
midway, and, lastly, Pisces is just beginning to set. In the Southeast, Cor 
Hydrae, Canis Minor, and Canis Major are conspicuous. In the South, 



314 GUIDE TO THE CONSTELLATIONS. 

Orion blazes brilliantly. In the Southwest, Cetus is hiding below the 
horizon. North of the Ecliptic, Auriga is in the zenith ; west are Per- 
seus and Andromeda, while Pegasus is just beginning to sink out of 
sight. 

April. (7 P. m.) — In the North, Ursa Major is above and to the 
east of Polaris ; opposite and to the west is Perseus, Draco below and 
to the east, Cepheus below and to the west, Cassiopeia west. In tlie 
East, Bootes (Arcturus) is not quite fully risen. Along the Ecliptic, Virgo 
(Spica) is rising, Leo midway, Cancer reaches to the meridian, Gemini 
is past, next Taurus, then Aries, and, lastly, Pisces just setting. In 
the Southeast, is the Crater (the Cup) ; Hydra stretches its long neck to 
the meridian. In the South, Canis Minor. In the Southwest, Sirius and 
Orion ; the Egyptian X (p. 229) can now be seen. North of the Ecliptic, 
and in the northeast, are Coma Berenices and Cor Caroli ; above 
Gemini and Taurus is Auriga, while Andromeda is just setting in the 
northwest. 

May. (8 p. m.) — In the A T orth, Ursa Major is above Polaris, Ursa 
Minor and Draco are east, Cepheus and Cassiopeia below, and Perseus 
is west. In the East, Lyra is rising, and Hercules is just up. Along 
the Ecliptic, Libra is just rising, Virgo is midway, Leo is on the me- 
ridian, Cancer is past, next Gemini, and lastly Taurus just setting. In 
the South, stretching east and west of the meridian, is Hydra, with the 
Crater and Corvus a little east. In the Southwest, are Cor Hydrae, Canis 
Major, and Canis Minor, while Orion is just setting in the west. North 
of the Ecliptic, in the east, above Hercules, are Corona Borealis (The 
Northern Crown), Bootes (Arcturus), Coma Berenices, and Cor Caroli, 
which stretch nearly to the meridian. In the Northwest, above Taurus 
and Perseus, is Auriga. 

June. (8 p. M.)—In the North, Ursa Major is above Polaris, Draco 
and Ursa Minor are east, Cepheus is below and east, and Cassiopeia 
directly below. In the East, Cygnus (the Cross) and Aquila are rising, 
Lyra and Taurus Poniatowskii are well up. Along the Ecliptic, Scorpio 
is rising, Libra is midway, Virgo on the meridian, Leo past, Cancer 
midway, Gemini next, and Taurus just setting. In the South, are Cor 
vus and the Crater a little past the meridian. In the Sotilhwest, is Cor 
Hydrae, and in the west Canis Minor is nearing the horizon. North 
of the Ecliptic, in the east, above Scorpio, is Hercules ; then Corona 
and Bootes, and, near the meridian, Cor Caroli, and Coma Berenices. 
In the Northwest, is Auriga, just coming to the horizon. 

«Tuly. (9 P. M.) — In the North, Draco and Ursa Minor are above Pola- 
ris, Ursa Major is west, Cepheus east, and Cassiopeia below to the east. 
In the East, the Dolphin (Job's Coffin) is now well up, Cygnus is almost 



GUIDE TO THE CONSTELLATIONS. 315 

midway to the meridian, and Lyra is still higher. Along the Ecliptic, 
Capricornus is rising, Sagittarius (the Archer) is next, Scorpio, with 
its long tail swinging along the horizon, is directly south, Libra is past 
the meridian, Virgo midway, and Leo has almost reached the horizon. 
In the Southivest, the Crater is setting, and Corvus is just above. North 
of the Ecliptic, above Scorpio and east of the meridian, are Serpentarius, 
Hercules, and Taurus Poniatowskii ; Corona is almost on the meridian, 
to the west of which lie Bootes, Cor Caroli, and Coma Berenices. 

August. (9 p. m.) — In the North, Draco and Ursa Minor are above 
Polaris, Cepheus is above and to the east, Cassiopeia east, and Ursa 
Major west. In the Northeast, Perseus is just rising, while south of it, 
Andromeda and Pegasus are fairly up. Along the Ecliptic, Aquarius is 
risen, next Capricornus, Sagittarius reaches to the meridian, Scorpio is 
just past, Libra next, and Virgo (Spica) just touches the horizon. North 
of the Ecliptic, Taurus Poniatowskii is on and Lyra is just east of the 
meridian ; the Swan and Dolphin are east of Lyra, Serpentarius and 
Hercules are above Scorpio, and just west of the meridian ; thence west 
are Corona and Bootes, while far in the northwest are Coma Berenices 
and Cor Caroli. 

September. (8 p. m.) — Draco is above and to the west of Polaris, 
Cepheus above and to the east, Cassiopeia east, Ursa Major is below 
and to the west. In the Northeast, Perseus is just rising. In the East, 
Andromeda is fairly up, Pegasus is nearly midway to the meridian. 
Along the Ecliptic, Pisces is just rising, next Aquarius, Capricornus in 
the southwest, Sagittarius on the meridian in the south, next Scorpio in 
the southwest, Libra, and, lastly, Virgo just setting. North of the Ecliptic, 
Lyra is on the meridian, Cygnus, the Dolphin, and Aquila are just to 
the east ; while to the west, are Taurus Poniatowskii and Serpentarius ; 
north of these latter are Hercules, Corona, Bootes, Cor Caroli, and 
Coma Berenices. 

October, (7 p. m.) — In the North, Cepheus and Draco are above 
Polaris, Ursa Minor is west, Cassiopeia east, and Ursa Major below and 
west. In the Northeast, Perseus is fairly risen. In the East, Androm- 
eda is nearly midway to the zenith. Along the Ecliptic, Aries is just 
rising, Pisces well up, Aquarius and Capricornus are in the southeast, 
Sagittarius is in the south, Scorpio far down in the southwest, and Libra 
just setting. North of the Ecliptic, Cygnus and Aquila are on the me- 
ridian ; the Dolphin is just east of it, and far south ; Lyra is west of the 
meridian ; Taurus Poniatowskii is lower down and to the south ; Ser- 
pentarius is just above Scorpio ; next, in line with Scorpio and Polaris, 
is Hercules ; Corona and Bootes are toward the northwest, where 
Coma Berenices is just setting. 



316 GUIDE TO THE CONSTELLATIONS. 

November. (7 P. M.)—In the North, Ursa Major is below Folaris, 
Ursa Minor and Draco are to the west, Cepheus is above, and Cassiopeia 
above and to the east. In the Northeast, Auriga is just rising, and 
Perseus is above, nearly midway to the meridian. Along the Ecliptic, 
Taurus is just rising, next are Aries and Pisces ; Aquarius is on the me- 
ridian, south ; then Capricornus, and lastly Sagittarius, in the southwest. 
North of the Ecliptic, Pegasus and Andromeda lie east of the meridian, 
the Swan, Dolphin, Eagle, Taurus Poniatowskii, and Lyra west. In 
the Northwest, are Hercules and Corona. 

December. (7 p. m.) — In the North, Cassiopeia is above Polaris, 
Cepheus above and to the west, Perseus above and to the east, Draco 
west, and Ursa Major below. In the Northeast, below Perseus, is 
Auriga. In the East, Orion is rising. Along the Ecliptic, Gemini is 
just rising, Taurus is nearly midway, next Aries, Pisces is on the me 
ridian, then Aquarius, and lastly Capricornus, far in the southwest. 
In the South, east of the meridian, is Cetus, and west is Fomalhaut. 
North of the Ecliptic, Andromeda is nearly on the meridian, and Pega 
sus west of it ; Cygnus, Delpbinus, Lyra, and Aquila are about midway 
while Taurus Poniatowskii is just sinking to the horizon. In the North 
west, Hercules is just setting. 

Note. — It should be borne in mind that a month makes a variation of about two 
hours (30°) in the rise of a star ; hence, in the foregoing " Guide," the "January Sky" 
of 9 p. m. would be about the same as the " February Sky " of 7 p. m. ; the "January 
Sky" of 11 p. m. would be about the same as the " March Sky" of 7 p. m., &c. In 
this way the "Guide" may be used for any hour in the- night. The pupil will see 
that in the "Guide" the prominent figures and stars in each constellation are given 
in parentheses. Examples : the " Y " in Aquarius, the " scalene triangle" in Aries, 
"Job's Coffin" in the Dolphin, " Procyon" in Canis Minor, &c. These. aid in iden- 
tifying the constellation. 



APPARATUS. 




TO ILLUSTRATE PRECESSION 

of the Equinoxes,* make the 
simple apparatus shown in the 
cut. It represents Fig. 3S, 
and the explanation of that 
figure and several subsequent 
ones applies to it. The in- 
genuity of pupil and teacher 
will devise methods of explain- 
ing by means of this instru- 
ment many otherwise abstruse 
points under this difficult sub- 
ject. The following are sug- 
gestive merely : 

1. To show motion of earth's 
axis around pole of Ecliptic. — 
Move P, axis of earth's plate, 

around D F, whose circumference roughly represents the little circle 
(ellipse) described by the pole of the earth. (Fig. 41.) 

2. To 'show change of Polar Star. — The pupil can readily see that the 
north pole of the earth will, at different times, point to different stars 
located around this circle. Now, Polaris; next, Lyra. 

3. To show why present polar distance will gradually diminish and then 
increase (p. 217). — The polar star lies at a little distance from this circle 
(edge of plate) and the pole is gradually approaching the star, but will 
pass it and then recede further from it, until, finally, Lyra, lying 5 from 
this circle, will become the polar constellation. 

4. To show Precession of Eqttinoxes. — Pass axis of earth around small 

* The above apparatus was devised by Solomon Sias, A.M. M.D., Principal of 
Schoharie Union School, N. Y. It can be made by any ingenious pupil. The plates 
are cut out of tin ; the standard may be made with the knife or scroll-saw to suit 
one's taste ; the earth is half of a little wooden ball balanced on the wire pin C ; and 
the semicircle, poles, etc., are of wire. The different parts may be soldered or 
fastened together with tacks. 



318 APPARATUS. 

circle. Note the position of the equinoxes before moving, and their 
gradual change of position along the ecliptic. 

5. To show cause of Precession. — Apply explanation of Fig. 39. 

6. To show necessity for new stellar niafts occasionally , or careful reduc- 
tions to previous standards. With the change of equinoxes, there is 
also a change of the equinoctial system, p. 27. 

7. To illustrate Fig. 40. — Remove the wire semicircle, and, inclining 
the axis of the earth, spin the wire between the thumb and finger like 
a top. The equinoxes will pass around the ecliptic as they did when 
the axis was carried around in the previous experiments. 

Letting G B E represent the plane of the ecliptic, and G E the line 
of the equinoxes, we can use this apparatus to illustrate the seasons, 
etc., (p 95). Also, by placing a lamp near S, the phenomenon of day 
and night, long summer days, short winter days, etc. (pp. 97, etc.), can 
be easily explained. 



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INDEX. 



PAGE 

Aberration 117 

Aerolites 175 

Aldebaran ( Al deb'-a-ran) . . 225 

Algol 242 

Altitude .». 27 

Amplitude 27 

Anaxagoras 8 

Anaximander 7 

Andromeda 223 

Autinous and Eagle 236 

Aphelion 18 

Apogee 122 

Apparent motion 85 

Apsides Ill 

Arcturus 205, 232 

Argo 239 

Aries 224 

Asteroids 154 

Astrology 13 

Astronomy, Definition of 1 

" Antiquity of 5 

Auriga (Au-ri'-ga) 225 

Azimuth 27 

Baily's Beads 141 

Bella'trix 228 

Berenice's Hair (Coma Berenices) 231 

Betelgeuse (Be-tel'-je-uze) 228 

Biela's Comet 1S4, 307 

Bode's Law 154 

Bootes (Bo-O'-tes) 232 

Calendar, The 289 

Canes Venatici 231 

Canis Major, Canis Minor 229 

Cancer 229 

Capricornus .236 

Cassiopeia (KaV-se-o-pe'-e-a) 220 

Castor and Pollux 227 

Celestial Chemistry 258 

" Longitude 30 

" Measurement* 271 

". Pole 23 

" Sphere 24 



PAGE 

Centaur 236 

Cepheus 219 

Ceres 155 

Cetus 226 

Chaldeans 6 

Chinese G 

Chromosphere, The 53 

Comres (Co-lure' ; plu. Co-hires') 27 

Comets 185 

Conjunction 00, 64 

" Inferior 64 

" Superior 64 

Constellations 203 

Copernican System 14 

Cor Caroli . 231 

Corona of Sun C3, 141, 143, S03 

Cross, The Southern 239 

Crystalline Spheres S 

Cygni, No . 61 204, 241 

Cygnus 237 

Day and Night 87 

'' Change in Length of 98 

11 Civil, The 267 

c: Sidereil 263 

" Solar 264 

Declination 27, 29 

Dipper 216 

Diurnal Motion 87 

Dolphin 237 

Draco 218 

DryMoon 131 

Earth S2 

" Eccentricity of Orbit Gl 

" Rotation of S3 

" Rotundity of 85 

" Yearly Motion of 91 

Earth-shine 127 

Eccentricity 57 

Eclipses 138 

Ecliptic 29, 04 

" Obliquity of 29, 95 

" Plane cf 58 



324 



PAGE 

Ecliptic, Poles of 30 

Egyptians 8 

Ellipse 17, 57 

Elongation 64 

Equinoxes 28, 30, 98 

" Precession of 104 

" Vernal 104 

Equinoctial 27 

Eudoxus 8 

Evening Star 65, 70 

Faculae 50 

Fixed Stars, The 204 

" Distance of 204 

" Motion of 205,261 

" Names of 208 

" Parallax of 204 

Flames, Solar 262 

Focus 17,57 

Galaxy 253 

Galileo (Gal-i-lee-o) 19 

Gemini 226 

Geocentric... 64 

Gibbous 127 

Golden Number 145 

Granules 50 

Gravitation 24 

Grecians 6 

Greek Alphabet 208 

Gregorian Calendar 269 

Hall, Prof. Asaph l"2-3 

Hare 228 

Harvest Moon 130 

Heliocentric 64 

Hercules 233 

Herschel 170 

Herschel's Theory 254 

Hipparcbus 8 

Horizon « 26 

Horoscope 13 

Hour Circles 27 

Hyades 224 

Hydra 231 

Inferior Planets 63 

Interior " 56 

Irradiation 123 

Job's Coffin 237 

Jupiter 157 

Kepler 15 



Kepler's Laws 

Kirchhoffs Theory (Klrk'hof).. 



PAGE 
... 15 



Latitude, To Calculate 218, 281 

Leo 229 

Libra 235 

Librations 124 

Light, Aberration of 117 

" Refraction of 112 

" Velocity of 162 

Longitude, To Calculate 280 

Lunarians, The 126 

Lyra 238 

Magellanic Cloud*. 253 

Mars 150 

Mean Day 264 

Mercury 71 

Meridian 26 

Meteoroids 1S2 

Meteors 175 

Metonic Cycle 144 

Milk Dipper 236 

Milky Way 253 

Minor Planets 154 

Mira 242 

Moon 122 

" Differential Effect of 148 

" Eclipse of 145 

Morning Star 65, 70 

Motion, Apparent 85 

" Diurnal 87 

" of Star 205,261 

Nadir 26 

Naos 239 

Nebular Hypothesis 255 

Nebulae 246 

" Spectra of 262 

Neptune 172 

Newton 21 

Nodes 58,132 

Noon-mark 265 

North Polar Star 217 

Nucleus 53, 186 

Nutation 109 

Occultation 132 

Ophiucus 234 

Opposition 67 

Orbit of Planets 57 

" " Solar System 206 

Orion (O-ri-on) " . 227 



325 



PAGE 

Parallax 119 

M Annual 121 

" Change of Solar 279 

" Horizontal 121 

" Lunar 273 

k ' Solar 36,121,275 

Pegasus (Peg'-a-sus) 223 

Penumbra 51,139 

Perigee 122 

Perihelion 18 

Perseus 221 

Phases 66 

Photosphere 51 

Pisces 226 

Planets 55 

" Are they inhabited ? 61 

" Definition of 2 

" Size of 59 

Pleiades (Ple'-ya-dez) 225 

Polaris 217 

Polar Star. 217 

" distance 29 

Precession of Equinoxes 104 

Procyon (Pr6-cy-on) 229 

Protuberances, Solar 53 

Ptolemaic Theory 9 

Ptolemy 9 

Pythagoras 7 



Quadrature . 



Refraction 112 

Regulus 229 

Retrograde motion 65 

Right ascension 29 

Rising and setting 86 



Sagittarius , 

Saracens 

Saros 6, 

Saturn 

Scintillation 

Scorpio 

Seasons 

Serpenrarius 

Shooting Stars 

Sidereal Revolution 45. 

Sidereal System. 

Signs, Zodiacal 31, 106, 

Signs and Constellations not agreeing 

Sirius 204,229, 

Solar System 

" Motion of 



PAGE 

Solar time 264 

Solstices 30, 98 

Space 24 

Spectra 258 

Spectrum Analysis 258 

Spectroscope 259 

Spica 230 

Stars, The 203 

" Colored 241 

" Distance of 204 

" Diurnal Orbits of 89 

" Double 239 

" Number of 206 

" Proper motion of 205, 261 

11 Size 207 

" Temporary 243 

" Variable 242 

Sun 36 

" Change in form and place of 114 

" Diurnal motion of 87 

" Eclipse of 138 

" Heatof 54 

" Pathof 94 

' ' Protuberances of 53, 262 

" Yearly Path of 94 

" Spots 40 

Superior Planets 63 

Syzygies 149 

Synodic Revolution 45, 69 

Tauru s 224 

" Poniatowskii 234 

Thales 6 

Thuban 219 

Tides 147 

Time 263 

Top 109 

Transit 65 

Transit of Venus 277 

Triangles 224 

Twilight 116 

Twinkling 207 

Tycho Brahe (Bra or Bra) 15 

Umbra 138 

Uranus (U'-ra-nus) 170 

Ursa Major 215 

" Minor....." 217 

Vega 217,238 

Venus 77 

" Transit of 277 

Vertical Circle. 26 



326 



INDEX. 



PAGE 

Velocity of Light 162 

Virgo 230 

Vulcan 71 

Wet Moon 131 

Wilson's Theory 51 

Tear, The 268 



PAGE 

Year, Anomalistic 268 

" of Astronomers . 111,301 

" Sidereal 268 

" Tropical 268 

Zenith 26 

Zodiac (Zo'-di-5c) 31, 210 

Zodiacal Light (Zo-dl'-ac-al) 196 



